[DirectX] Strip loop metadata that will trip up the validator when generating DXIL

[bogner]

Compiling particle_life with clang fails the validator with an error about llvm.loop.mustprogress being invalid. This particular metadata node is newer than the version of LLVM that DXIL is based off of, so it makes sense that it's a problem.

We need to strip metadata that the DXIL validator (and thus, potentially, DXIL drivers) isn't aware of. This may be as simple as the following:

--- a/llvm/lib/Target/DirectX/DXILPrepare.cpp
+++ b/llvm/lib/Target/DirectX/DXILPrepare.cpp
@@ -189,6 +189,26 @@ public:
       for (auto &BB : F) {
         IRBuilder<> Builder(&BB);
         for (auto &I : make_early_inc_range(BB)) {
+
+          // TODO: Audit this list - is it enough? Too much?
+          static unsigned DXILCompatibleMDs[] = {
+              LLVMContext::MD_dbg,
+              LLVMContext::MD_tbaa,
+              LLVMContext::MD_prof,
+              LLVMContext::MD_fpmath,
+              LLVMContext::MD_range,
+              LLVMContext::MD_tbaa_struct,
+              LLVMContext::MD_invariant_load,
+              LLVMContext::MD_alias_scope,
+              LLVMContext::MD_noalias,
+              LLVMContext::MD_nontemporal,
+              LLVMContext::MD_mem_parallel_loop_access,
+              LLVMContext::MD_nonnull,
+              LLVMContext::MD_dereferenceable,
+              LLVMContext::MD_dereferenceable_or_null,
+          };
+          I.dropUnknownNonDebugMetadata(DXILCompatibleMDs);
+
           if (I.getOpcode() == Instruction::FNeg) {
             Builder.SetInsertPoint(&I);
             Value *In = I.getOperand(0);

Here, we strip anything that isn't listed in DXC's set of pinned metadata names in LLVMContext.h. We need to determine if this is the right granularity.

[bogner]

DXV's handling of instruction level metadata is here: https://github.com/microsoft/DirectXShaderCompiler/blob/8280d0fb4104ce9c67af3b9f4a1335760c7c5113/lib/DxilValidation/DxilValidation.cpp#L2606-L2633