Add default value for MachineInstr::modifiesRegister. NFC.

[chill]

Summary

Looks accidentally omitted, it's present on readsRegister, definesRegister and few others.

[chill]

Ping.

[yroux]

I'd check if there are some calls of modifiesRegister that pass nullptr, the patch LGTM otherwise

[chill]

In D89625#2367785, @yroux wrote:

I'd check if there are some calls of modifiesRegister that pass nullptr, the patch LGTM otherwise

AFAICT, there's one such call, in the Hexagon backend, but I don't think we should change it (or other places) to take advantage of the default value.

[yroux]

LGTM

[chill]

Cheers!

llvm/include/llvm/CodeGen/MachineInstr.h

   bool definesRegister(Register Reg,
                        const TargetRegisterInfo *TRI = nullptr) const {
     return findRegisterDefOperandIdx(Reg, false, false, TRI) != -1;
   }
 
   /// Return true if the MachineInstr modifies (fully define or partially
   /// define) the specified register.
   /// NOTE: It's ignoring subreg indices on virtual registers.
-  bool modifiesRegister(Register Reg, const TargetRegisterInfo *TRI) const {
+  bool modifiesRegister(Register Reg,
+                        const TargetRegisterInfo *TRI = nullptr) const {
     return findRegisterDefOperandIdx(Reg, false, true, TRI) != -1;
   }
 
   /// Returns true if the register is dead in this machine instruction.
   /// If TargetRegisterInfo is passed, then it also checks
   /// if there is a dead def of a super-register.
   bool registerDefIsDead(Register Reg,
                          const TargetRegisterInfo *TRI = nullptr) const {