[PGO] Add MD_prof into known-id set when combining metadata. The implementation of `combineMetadata` drops unknown types of metadata.

[mingmingl]

Summary

• Tests to be added.

llvm/lib/Transforms/Scalar/GVNHoist.cpp

 static void combineKnownMetadata(Instruction *ReplInst, Instruction *I) {
   static const unsigned KnownIDs[] = {LLVMContext::MD_tbaa,
                                       LLVMContext::MD_alias_scope,
                                       LLVMContext::MD_noalias,
                                       LLVMContext::MD_range,
                                       LLVMContext::MD_fpmath,
                                       LLVMContext::MD_invariant_load,
                                       LLVMContext::MD_invariant_group,
+                                      LLVMContext::MD_prof,
                                       LLVMContext::MD_access_group};
   combineMetadata(ReplInst, I, KnownIDs, true);
 }
 
 // This pass hoists common computations across branches sharing common
 // dominator. The primary goal is to reduce the code size, and in some
 // cases reduce critical path (by exposing more ILP).
 class GVNHoist {

llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp

MSSAU->moveBefore(MSSA->getMemoryAccess(SkippedLifetimeStart),
                       MSSA->getMemoryAccess(C));
   }
 
   // Update AA metadata
   // FIXME: MD_tbaa_struct and MD_mem_parallel_loop_access should also be
   // handled here, but combineMetadata doesn't support them yet
   unsigned KnownIDs[] = {LLVMContext::MD_tbaa, LLVMContext::MD_alias_scope,
                          LLVMContext::MD_noalias,
+                         LLVMContext::MD_prof,
                          LLVMContext::MD_invariant_group,
                          LLVMContext::MD_access_group};
   combineMetadata(C, cpyLoad, KnownIDs, true);
   if (cpyLoad != cpyStore)
     combineMetadata(C, cpyStore, KnownIDs, true);
 
   ++NumCallSlot;
   return true;