Clang ICE in SelectionDAG when indexing extremely large vector_size type

### Link for quick verification: https://godbolt.org/z/ncTPnj3f9
```c
typedef int v4si __attribute__ ((vector_size (1 << 29)));
void foo(v4si x) {
x[0] = 1;
}
```
Go back to `clang 11.0.0 (assertions)`.
Using an extremely large GCC-style vector type (e.g. `__attribute__((vector_size(1<<29)))`) and performing a simple element access
(`x[0] = 1`) triggers an ICE, which indicates a mismatch between the computed memory access size for vector element indexing and the associated `MachineMemOperand` size when handling vectors with extremely large total sizes.
## Stack dump
```console
clang++: /root/llvm-project/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp:13105: llvm::MemSDNode::MemSDNode(unsigned int, unsigned int, const llvm::DebugLoc&, llvm::SDVTList, llvm::EVT, llvm::MachineMemOperand*): Assertion `(!MMO->getType().isValid() || TypeSize::isKnownLE(memvt.getStoreSize(), MMO->getSize().getValue())) && "Size mismatch!"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0. Program arguments: /opt/compiler-explorer/clang-assertions-trunk/bin/clang++ -g -o /app/output.s -mllvm --x86-asm-syntax=intel -fno-verbose-asm -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics
1. parser at end of file
2. Code generation
3. Running pass 'Function Pass Manager' on module ''.
4. Running pass 'X86 DAG->DAG Instruction Selection' on function '@_Z3fooDv134217728_i'
#0 0x0000000004301458 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4301458)
#1 0x00000000042fe884 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x42fe884)
#2 0x0000000004243628 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#3 0x0000726a02a42520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#4 0x0000726a02a969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
#5 0x0000726a02a42476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
#6 0x0000726a02a287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
#7 0x0000726a02a2871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
#8 0x0000726a02a39e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
#9 0x00000000058dcb9c llvm::MemSDNode::MemSDNode(unsigned int, unsigned int, llvm::DebugLoc const&, llvm::SDVTList, llvm::EVT, llvm::MachineMemOperand*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x58dcb9c)
#10 0x00000000058eca01 llvm::SelectionDAG::getLoad(llvm::ISD::MemIndexedMode, llvm::ISD::LoadExtType, llvm::EVT, llvm::SDLoc const&, llvm::SDValue, llvm::SDValue, llvm::SDValue, llvm::EVT, llvm::MachineMemOperand*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x58eca01)
#11 0x0000000005901f9d llvm::SelectionDAG::getLoad(llvm::ISD::MemIndexedMode, llvm::ISD::LoadExtType, llvm::EVT, llvm::SDLoc const&, llvm::SDValue, llvm::SDValue, llvm::SDValue, llvm::MachinePointerInfo, llvm::EVT, llvm::Align, llvm::MachineMemOperand::Flags, llvm::AAMDNodes const&, llvm::MDNode const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5901f9d)
#12 0x000000000590283f llvm::SelectionDAG::getLoad(llvm::EVT, llvm::SDLoc const&, llvm::SDValue, llvm::SDValue, llvm::MachinePointerInfo, llvm::MaybeAlign, llvm::MachineMemOperand::Flags, llvm::AAMDNodes const&, llvm::MDNode const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x590283f)
#13 0x00000000058bd985 llvm::SelectionDAGBuilder::visitLoad(llvm::LoadInst const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x58bd985)
#14 0x00000000058d0502 llvm::SelectionDAGBuilder::visit(llvm::Instruction const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x58d0502)
#15 0x0000000005962fcd llvm::SelectionDAGISel::SelectBasicBlock(llvm::ilist_iterator_w_bits, false, true>, llvm::ilist_iterator_w_bits, false, true>, bool&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5962fcd)
#16 0x0000000005963a38 llvm::SelectionDAGISel::SelectAllBasicBlocks(llvm::Function const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5963a38)
#17 0x000000000596556c llvm::SelectionDAGISel::runOnMachineFunction(llvm::MachineFunction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x596556c)
#18 0x0000000005950dff llvm::SelectionDAGISelLegacy::runOnMachineFunction(llvm::MachineFunction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5950dff)
#19 0x0000000003598cf9 llvm::MachineFunctionPass::runOnFunction(llvm::Function&) (.part.0) MachineFunctionPass.cpp:0:0
#20 0x0000000003bef316 llvm::FPPassManager::runOnFunction(llvm::Function&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3bef316)
#21 0x0000000003bef6c1 llvm::FPPassManager::runOnModule(llvm::Module&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3bef6c1)
#22 0x0000000003beff2f llvm::legacy::PassManagerImpl::run(llvm::Module&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3beff2f)
#23 0x00000000045c2e3c (anonymous namespace)::EmitAssemblyHelper::RunCodegenPipeline(clang::BackendAction, std::unique_ptr>&, std::unique_ptr>&) BackendUtil.cpp:0:0
#24 0x00000000045c3770 clang::emitBackendOutput(clang::CompilerInstance&, clang::CodeGenOptions&, llvm::StringRef, llvm::Module*, clang::BackendAction, llvm::IntrusiveRefCntPtr, std::unique_ptr>, clang::BackendConsumer*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x45c3770)
#25 0x0000000004c62327 clang::BackendConsumer::HandleTranslationUnit(clang::ASTContext&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4c62327)
#26 0x0000000006a1e5ac clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a1e5ac)
#27 0x0000000004c62788 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4c62788)
#28 0x0000000004f58e05 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f58e05)
#29 0x0000000004ed909e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4ed909e)
#30 0x000000000505318d clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x505318d)
#31 0x0000000000de63ec cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xde63ec)
#32 0x0000000000ddce3a ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#33 0x0000000000ddcfbd int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#34 0x0000000004cd4e59 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#35 0x0000000004243ac4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4243ac4)
#36 0x0000000004cd546f clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#37 0x0000000004c95f52 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4c95f52)
#38 0x0000000004c96efe clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4c96efe)
#39 0x0000000004c9e355 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4c9e355)
#40 0x0000000000de27e1 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xde27e1)
#41 0x0000000000c8d114 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc8d114)
#42 0x0000726a02a29d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#43 0x0000726a02a29e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#44 0x0000000000ddc8d5 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xddc8d5)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134
```

Reproduce the crash with the provided C vector example, then inspect llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp around MemSDNode and SelectionDAG::getLoad, along with SelectionDAGBuilder::visitLoad from the stack trace. Done means the reproducer no longer triggers the assertion and regression coverage verifies the behavior.

Written by the indexing model from the issue text.