AMDGPU: Fold frame index into MUBUF

This matters for byval uses outside of the entry block, which appear
as copies.

Previously, the only folding done was during selection, which could
not see the underlying frame index. For any uses outside the entry
block, the frame index was materialized in the entry block relative to
the global scratch wave offset.

This may produce worse code in cases where the offset ends up not
fitting in the MUBUF offset field. A better heuristic would be helpfu
for extreme frames.

llvm-svn: 364185