[AMDGPU] Check if type transforms to i16 (VI+) when getting AMDGPUISD::FFBH_U32

[AMDGPU] Check if type transforms to i16 (VI+) when getting AMDGPUISD::FFBH_U32

This will prevent following regression when enabling i16 support (D18049):

test/CodeGen/AMDGPU/ctlz.ll
test/CodeGen/AMDGPU/ctlz_zero_undef.ll

Differential Revision: https://reviews.llvm.org/D25802

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@285716 91177308-0d34-0410-b5e6-96231b3b80d8