AMDGPU/SI: Fix commuting of 32-bit VOPC instructions

Summary:
We didn't have entries in the commuting table for the 32-bit
instructions.  I don't think we hit this problem now, but we
will once uniform branching is enabled.  Tests will come in
a later commit.

Reviewers: arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D16600

llvm-svn: 258936