AMDGPU: Fix folding SGPRs into madak/madmk src0

Because of the special immediate operand, the constant
bus is already used so SGPRs are never useful.

r263212 changed the name of the immediate operand, which
broke the verifier check for the restriction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274564 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed