The AMDGPU backend was the last user of the old StringMatcher
recognition code. Move it over to the new lookupLLVMIntrinsicName
funciton, which is now improved to handle all of the interesting edge
cases exposed by AMDGPU intrinsic names.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@258875 91177308-0d34-0410-b5e6-96231b3b80d8