[TargetRegisterInfo, AArch64] Add target hook for isConstantPhysReg().

Summary:
The current implementation of isConstantPhysReg() checks for defs of
physical registers to determine if they are constant.  Some
architectures (e.g. AArch64 XZR/WZR) have registers that are constant
and may be used as destinations to indicate the generated value is
discarded, preventing isConstantPhysReg() from returning true.  This
change adds a TargetRegisterInfo hook that overrides the no defs check
for cases such as this.

Reviewers: MatzeB, qcolombet, t.p.northover, jmolloy

Subscribers: junbuml, aemerson, mcrosier, rengolin

Differential Revision: https://reviews.llvm.org/D24570

llvm-svn: 282543