[CodeGen] Print register names in lowercase in both MIR and debug output

[thegameg]

Summary

As part of the unification of the debug format and the MIR format, always print registers as lowercase.

• Only debug printing is affected. It now follows MIR.

[MatzeB]

LGTM, thanks. I've had one comment about the lowercase printing below. It's only a very minor issue, so feel free to fix this as part of this commit or separate it out into a separate patch.

lib/CodeGen/TargetRegisterInfo.cpp

97

This creates a temporary std::string object each time.

How about adding a printLowerCase(const char *String, raw_ostream &OS) function to StringExtras.h that transforms the characters to lowercase as they are printed? (May also want to move ascii_tolower() from StringRef.cpp to StringExtras.h to share code).

[thegameg]

Uploaded separately r319171:Merge toLower / toUpper implementations.

lib/CodeGen/TargetRegisterInfo.cpp

Printable printReg(unsigned Reg, const TargetRegisterInfo *TRI,
   return Printable([Reg, TRI, SubIdx](raw_ostream &OS) {
     if (!Reg)
       OS << "%noreg";
     else if (TargetRegisterInfo::isStackSlot(Reg))
       OS << "SS#" << TargetRegisterInfo::stackSlot2Index(Reg);
     else if (TargetRegisterInfo::isVirtualRegister(Reg))
       OS << "%vreg" << TargetRegisterInfo::virtReg2Index(Reg);
     else if (TRI && Reg < TRI->getNumRegs())
-      OS << '%' << TRI->getName(Reg);
+      OS << '%' << StringRef(TRI->getName(Reg)).lower();
     else
       OS << "%physreg" << Reg;
     if (SubIdx) {
       if (TRI)
         OS << ':' << TRI->getSubRegIndexName(SubIdx);
       else
         OS << ":sub(" << SubIdx << ')';
     }