[AArch64] Fold some filled/spilled subreg COPYs

Summary:
Extend AArch64 foldMemoryOperandImpl() to handle folding spills of
subreg COPYs with read-undef defs like:

  %vreg0:sub_32<def,read-undef> = COPY %WZR; GPR64:%vreg0

by widening the spilled physical source reg and generating:

  STRXui %XZR <fi#0>

as well as folding fills of similar COPYs like:

  %vreg0:sub_32<def,read-undef> = COPY %vreg1; GPR64:%vreg0, GPR32:%vreg1

by generating:

  %vreg0:sub_32<def,read-undef> = LDRWui <fi#0>

Reviewers: MatzeB, qcolombet

Subscribers: aemerson, rengolin, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D27425

llvm-svn: 291180