R600/SI: Legalize INSERT_SUBREG instructions during PostISelFolding

LLVM assumes INSERT_SUBREG will always have register operands, so
we need to legalize non-register operands, like FrameIndexes, to
avoid random assertion failures.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219420 91177308-0d34-0410-b5e6-96231b3b80d8