[CodeGenPrepare] hit unreachable on scalable non-splat constant promotion

### Reproducer
```llvm
target triple = "x86_64-unknown-linux-gnu"
define void @f(ptr %p) {
entry:
%v = insertelement poison, i32 1, i64 0
%e = extractelement %v, i64 0
%a = add i32 %e, 7
store i32 %a, ptr %p
ret void
}
```
```console
$ opt -codegenprepare -stress-cgp-store-extract repro.ll
Generate scalable vector for non-splat is unimplemented
UNREACHABLE executed at /root/llvm-project/llvm/lib/CodeGen/CodeGenPrepare.cpp:8370!
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and instructions to reproduce the bug.
Stack dump:
0. Program arguments: /opt/compiler-explorer/clang-assertions-trunk/bin/opt -o /app/output.s -S -codegenprepare -stress-cgp-store-extract
1. Running pass 'Function Pass Manager' on module ''.
2. Running pass 'CodeGen Prepare' on function '@f'
#0 0x0000000005c8cfd8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0x5c8cfd8)
#1 0x0000000005c8a234 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0x5c8a234)
#2 0x0000000005c8a3ae SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
#3 0x00007bd322645330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#4 0x00007bd32269ec0c pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x9ec0c)
#5 0x00007bd32264527e raise (/lib/x86_64-linux-gnu/libc.so.6+0x4527e)
#6 0x00007bd3226288ff abort (/lib/x86_64-linux-gnu/libc.so.6+0x288ff)
#7 0x0000000005bb62ba (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0x5bb62ba)
#8 0x00000000041b696c (anonymous namespace)::CodeGenPrepare::optimizeExtractElementInst(llvm::Instruction*) CodeGenPrepare.cpp:0:0
#9 0x00000000041dceb4 (anonymous namespace)::CodeGenPrepare::optimizeInst(llvm::Instruction*, (anonymous namespace)::ModifyDT&) CodeGenPrepare.cpp:0:0
#10 0x00000000041dd6c7 (anonymous namespace)::CodeGenPrepare::_run(llvm::Function&) CodeGenPrepare.cpp:0:0
#11 0x00000000041e033b (anonymous namespace)::CodeGenPrepareLegacyPass::runOnFunction(llvm::Function&) (.part.0) CodeGenPrepare.cpp:0:0
#12 0x00000000059b3964 llvm::FPPassManager::runOnFunction(llvm::Function&) (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0x59b3964)
#13 0x00000000059b3e61 llvm::FPPassManager::runOnModule(llvm::Module&) (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0x59b3e61)
#14 0x00000000059b4829 llvm::legacy::PassManagerImpl::run(llvm::Module&) (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0x59b4829)
#15 0x00000000008e7c36 optMain (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0x8e7c36)
#16 0x00007bd32262a1ca (/lib/x86_64-linux-gnu/libc.so.6+0x2a1ca)
#17 0x00007bd32262a28b __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28b)
#18 0x00000000008dd095 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0x8dd095)
Program terminated with signal SIGABRT (6)
Compiler returned: 134
```
Without assertions, it reports the broken IR from opt 16.0.0 to trunk (reproduced on godbolt).
godbolt: https://godbolt.org/z/ve4qhbxbv

Start in llvm/lib/CodeGen/CodeGenPrepare.cpp, especially optimizeExtractElementInst around the reported line, and run opt with -codegenprepare -stress-cgp-store-extract on the provided LLVM IR. Investigate the scalable non-splat constant-promotion path. Done means the reproducer no longer hits unreachable and opt does not emit broken IR.

Written by the indexing model from the issue text.