[AArch64] Fold spills of COPY of WZR/XZR

Summary:
In AArch64InstrInfo::foldMemoryOperandImpl, catch more cases where the
COPY being spilled is copying from WZR/XZR, but the source register is
not in the COPY destination register's regclass.

For example, when spilling:

  %vreg0 = COPY %XZR ; %vreg0:GPR64common

without this change, the code in TargetInstrInfo::foldMemoryOperand()
and canFoldCopy() that normally handles cases like this would fail to
optimize since %XZR is not in GPR64common.  So the spill code generated
would be:

  %vreg0 = COPY %XZR
  STR %vreg

instead of the new code generated:

  STR %XZR

Reviewers: qcolombet, MatzeB

Subscribers: mcrosier, aemerson, t.p.northover, llvm-commits, rengolin

Differential Revision: https://reviews.llvm.org/D26976

llvm-svn: 288176