[AArch64] Extend AArch64RedundantCopyElimination to do simple copy propagation.

Summary:
Extend AArch64RedundantCopyElimination to catch cases where the register
that is known to be zero is COPY'd in the predecessor block.  Before
this change, this pass would catch cases like:

      CBZW %W0, <BB#1>
  BB#1:
      %W0 = COPY %WZR // removed

After this change, cases like the one below are also caught:

      %W0 = COPY %W1
      CBZW %W1, <BB#1>
  BB#1:
      %W0 = COPY %WZR // removed

This change results in a 4% increase in static copies removed by this
pass when compiling the llvm test-suite.  It also fixes regressions
caused by doing post-RA copy propagation (a separate change to be put up
for review shortly).

Reviewers: junbuml, mcrosier, t.p.northover, qcolombet, MatzeB

Subscribers: aemerson, rengolin, llvm-commits

Differential Revision: https://reviews.llvm.org/D30113

llvm-svn: 295863