AMDGPU: Don't form fmed3 if it will require materialization

If there is a single use constant, it can be folded into the
min/max, but not into med3.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@342443 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed