[PGO] Enable memcmp/bcmp size value profiling.

[hjyamauchi]

Summary

Following up D79751.

[davidxl]

What is the profile size increase with this change (e.g with Clang's profile)?

[hjyamauchi]

~0.04% increase in Clang's profile.

Before 37554248
After 37570080 (1.000421x)

This flag only guarded the optimization side. I uploaded D80646 to also have the instrumentation side behind the flag before enabling it.

[hjyamauchi]

Rebase past D80646.

[davidxl]

lgtm

llvm/lib/Transforms/Instrumentation/PGOMemOPSizeOpt.cpp

 
 // This option sets the rangge of precise profile memop sizes.
 extern cl::opt<std::string> MemOPSizeRange;
 
 // This option sets the value that groups large memop sizes
 extern cl::opt<unsigned> MemOPSizeLarge;
 
 cl::opt<bool>
-    MemOPOptMemcmpBcmp("pgo-memop-optimize-memcmp-bcmp", cl::init(false),
+    MemOPOptMemcmpBcmp("pgo-memop-optimize-memcmp-bcmp", cl::init(true),
                        cl::Hidden,
                        cl::desc("Size-specialize memcmp and bcmp calls"));
 
 namespace {
 class PGOMemOPSizeOptLegacyPass : public FunctionPass {
 public:
   static char ID;