AMDGPU: Try to use function specific ST

Subtargets are a function level property, so ideally we would
eliminate everywhere that needs to check the global one. Rename the
function to try avoiding confusion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353900 91177308-0d34-0410-b5e6-96231b3b80d8