[AMDGPU] Supress unused sdwa insts generation

Do not generate non-existing sdwa instructions. It reduces the
number of generated instructions by 185.

Differential Revision: https://reviews.llvm.org/D69010

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375016 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed