GlobalISel: constrain PHI registers on AArch64.

Self-referencing PHI nodes need their destination operands to be constrained
because nothing else is likely to do so. For now we just pick a register class
naively.

Patch mostly by Ahmed again.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286183 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed