AMDGPU: Diagnose illegal SGPR to VGPR copies

This is possible in ways that are not compiler bugs,
so stop asserting on them.

This emits an extra error when emitting objects when it
can't encode the new pseudo, but I'm not sure that matters.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299712 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed