AMDGPU: Pattern match ffbh pattern to instruction.

AMDGPU: Pattern match ffbh pattern to instruction.

The hardware instruction's output on 0 is -1 rather than 32.
Eliminate a test and select to -1. This removes an extra instruction
from the compatability function with HSAIL's firstbit instruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257352 91177308-0d34-0410-b5e6-96231b3b80d8