[AMDGPU] Shrinking V_SUBBREV_U32

V_SUBBREV_U32 is a commute opcode for V_SUBB_U32. However, when
we try to commute V_SUBB_U32 in order to shrink it we do not then
process V_SUBBREV_U32 and it stay VOP3. This is fixed.

Differential Revision: https://reviews.llvm.org/D43699

llvm-svn: 326011