[ARM GlobalISel] Select G_PHI

Select G_PHI to PHI and manually constrain the result register. This is
very similar to how COPY is handled, so extract and reuse some of that
code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@321797 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed