Using large vert/hull/domain IO makes DXC crash when building to DXIL

Description

The issue was originally only on the SPIR-V backend (#3735), but while fixing it for SPIR-V, noticed DXIL is now also broken.

Steps to Reproduce

https://godbolt.org/z/T9c5Tdnsf

struct ControlPoint {
  float4 position[100] : POSITION;
};

struct HullPatchOut {
    float edge [3] : SV_TessFactor;
    float inside : SV_InsideTessFactor;
};

HullPatchOut HullConst () {
  return (HullPatchOut)0;
}

[domain("tri")]
[partitioning("fractional_odd")]
[outputtopology("triangle_cw")]
[patchconstantfunc("HullConst")]
[outputcontrolpoints(3)]
ControlPoint Hull(InputPatch<ControlPoint, 1> v,
                  uint id : SV_OutputControlPointID) {
  ControlPoint p;
  return p;
}

Actual Behavior

[user@hostname DirectXShaderCompiler]$ dxc -T hs_6_0 -E Hull repro.hlsl
Failed to allocate all input signature elements in available space.
UNREACHABLE executed at DirectXShaderCompiler/lib/HLSL/HLSignatureLower.cpp:523!
Aborted

Backtrace:

 thread #1, name = 'dxc', stop reason = signal SIGABRT
  * frame #0: 0x00007ffff3ca80fc libc.so.6`__pthread_kill_implementation(threadid=<unavailable>, signo=6, no_tid=<unavailable>) at pthread_kill.c:44:76
    frame #1: 0x00007ffff3c5a472 libc.so.6`__GI_raise(sig=6) at raise.c:26:13
    frame #2: 0x00007ffff3c444b2 libc.so.6`__GI_abort at abort.c:79:7
    frame #3: 0x00007ffff4c8dd8a libdxcompiler.so`llvm::llvm_unreachable_internal(msg="Failed to allocate all input signature elements in available space.", file="DirectXShaderCompiler/lib/HLSL/HLSignatureLower.cpp", line=523) at ErrorHandling.cpp:137:3
    frame #4: 0x00007ffff49449e7 libdxcompiler.so`hlsl::HLSignatureLower::AllocateDxilInputOutputs(this=0x00007fffffff79b0) at HLSignatureLower.cpp:522:5
    frame #5: 0x00007ffff494c834 libdxcompiler.so`hlsl::HLSignatureLower::Run(this=0x00007fffffff79b0) at HLSignatureLower.cpp:1797:5
    frame #6: 0x00007ffff48ccb62 libdxcompiler.so`(anonymous namespace)::DxilGenerationPass::runOnModule(this=0x0000555555ab7490, M=0x0000555555a32490) at DxilGenerationPass.cpp:223:16
    frame #7: 0x00007ffff67e83dd libdxcompiler.so`(anonymous namespace)::MPPassManager::runOnModule(this=0x0000555555a260f0, M=0x0000555555a32490) at LegacyPassManager.cpp:1669:27
    frame #8: 0x00007ffff67e7e86 libdxcompiler.so`llvm::legacy::PassManagerImpl::run(this=0x0000555555aa7160, M=0x0000555555a32490) at LegacyPassManager.cpp:1771:44
    frame #9: 0x00007ffff67e8a94 libdxcompiler.so`llvm::legacy::PassManager::run(this=0x0000555555aa70d0, M=0x0000555555a32490) at LegacyPassManager.cpp:1814:14
    frame #10: 0x00007ffff4dcddec libdxcompiler.so`(anonymous namespace)::EmitAssemblyHelper::EmitAssembly(this=0x00007fffffff8248, Action=Backend_EmitBC, OS=0x0000555555a319f0) at BackendUtil.cpp:756:22
    frame #11: 0x00007ffff4dcd14e libdxcompiler.so`clang::EmitBackendOutput(Diags=0x00005555559f59a0, CGOpts=0x00005555559f44c0, TOpts=0x00005555559f5080, LOpts=0x00005555559f4f10, TDesc=(Data = "e-m:e-p:32:32-i1:32-i8:32-i16:32-i32:32-i64:64-f16:32-f32:32-f64:64-n8:16:32:64", Length = 79), M=0x0000555555a32490, Action=Backend_EmitBC, OS=0x0000555555a319f0) at BackendUtil.cpp:779:15
    frame #12: 0x00007ffff4da4bf9 libdxcompiler.so`clang::BackendConsumer::HandleTranslationUnit(this=0x0000555555a31ae0, C=0x0000555555a1bc60) at CodeGenAction.cpp:191:7
    frame #13: 0x00007ffff6393392 libdxcompiler.so`clang::ParseAST(S=0x0000555555a327b0, PrintStats=false, SkipFunctionBodies=false) at ParseAST.cpp:164:13
    frame #14: 0x00007ffff515dc9c libdxcompiler.so`clang::ASTFrontendAction::ExecuteAction(this=0x00007fffffff9c20) at FrontendAction.cpp:554:3
    frame #15: 0x00007ffff4da30c2 libdxcompiler.so`clang::CodeGenAction::ExecuteAction(this=0x00007fffffff9c20) at CodeGenAction.cpp:807:28
    frame #16: 0x00007ffff515d78c libdxcompiler.so`clang::FrontendAction::Execute(this=0x00007fffffff9c20) at FrontendAction.cpp:455:8
    frame #17: 0x00007ffff43f05c6 libdxcompiler.so`DxcCompiler::Compile(this=0x00005555559f19e0, pSource=0x00007fffffffc270, pArguments=0x00005555559f1930, argCount=5, pIncludeHandler=0x00005555559f16c0, riid=0x00005555558aecc0, ppResult=0x00007fffffffb7b8) at dxcompilerobj.cpp:972:18
    frame #18: 0x00007ffff43e4d47 libdxcompiler.so`hlsl::DxcCompilerAdapter::WrapCompile(this=0x00005555559f1e00, bPreprocess=NO, pSource=0x00005555559f1e90, pSourceName=L"repro.hlsl", pEntryPoint=L"Hull", pTargetProfile=L"hs_6_0", pArguments=0x00005555559f1980, argCount=4, pDefines=0x00007fffffffced8, defineCount=0, pIncludeHandler=0x00005555559f16c0, ppResult=0x00007fffffffca70, ppDebugBlobName=0x0000000000000000, ppDebugBlob=0x0000000000000000) at dxcompilerobj.cpp:1820:5
    frame #19: 0x00007ffff43e624c libdxcompiler.so`hlsl::DxcCompilerAdapter::CompileWithDebug(this=0x00005555559f1e00, pSource=0x00005555559f1e90, pSourceName=L"repro.hlsl", pEntryPoint=L"Hull", pTargetProfile=L"hs_6_0", pArguments=0x00005555559f1980, argCount=4, pDefines=0x00007fffffffced8, defineCount=0, pIncludeHandler=0x00005555559f16c0, ppResult=0x00007fffffffca70, ppDebugBlobName=0x0000000000000000, ppDebugBlob=0x0000000000000000) at dxcompilerobj.cpp:1723:10
    frame #20: 0x00007ffff43ea3b6 libdxcompiler.so`hlsl::DxcCompilerAdapter::Compile(this=0x00005555559f1e00, pSource=0x00005555559f1e90, pSourceName=L"repro.hlsl", pEntryPoint=L"Hull", pTargetProfile=L"hs_6_0", pArguments=0x00005555559f1980, argCount=4, pDefines=0x00007fffffffced8, defineCount=0, pIncludeHandler=0x00005555559f16c0, ppResult=0x00007fffffffca70) at dxcompileradapter.h:75:12
    frame #21: 0x00005555555c5ccc dxc`DxcContext::Compile(this=0x00007fffffffcd38) at dxc.cpp:873:9
    frame #22: 0x00005555555c9ee1 dxc`dxc::main(argc=6, argv_=0x00007fffffffd9c8) at dxc.cpp:1501:24
    frame #23: 0x00005555555be512 dxc`main(argc=6, argv_=0x00007fffffffd9c8) at dxcmain.cpp:19:10
    frame #24: 0x00007ffff3c456ca libc.so.6`__libc_start_call_main(main=(dxc`main at dxcmain.cpp:18), argc=6, argv=0x00007fffffffd9c8) at libc_start_call_main.h:58:16
    frame #25: 0x00007ffff3c45785 libc.so.6`__libc_start_main_impl(main=(dxc`main at dxcmain.cpp:18), argc=6, argv=0x00007fffffffd9c8, init=0x00007ffff7ffd000, fini=<unavailable>, rtld_fini=<unavailable>, stack_end=0x00007fffffffd9b8) at libc-start.c:360:3
    frame #26: 0x00005555555be421 dxc`_start + 33

Environment

• DXC version HEAD (bdd7d9b0b9e8bfd28d6f25e8287f4a5fcd0d864c)
• Host Operating System: Linux

Start with the HLSL input-signature allocation path in lib/HLSL/HLSignatureLower.cpp, especially AllocateDxilInputOutputs around line 523. Reproduce the failure with the linked Godbolt example or the dxc hs_6_0 command and inspect the reported assertion. Done means the large hull/domain input compiles without crashing and preserves the expected diagnostic or output behavior.