AMDGPU: Undo sub x, c -> add x, -c canonicalization

This is worse if the original constant is an inline immediate.

This should also be done for 64-bit adds, but requires fixing
operand folding bugs first.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293540 91177308-0d34-0410-b5e6-96231b3b80d8