[AMDGPU] Fix lowering of AMDGPU::G_CTTZ_ZERO_UNDEF to AMDGPU::G_AMDGPU_FFBL_B32

As mentioned on D107474, there was a copy+paste typo repeating G_CTLZ_ZERO_UNDEF that coverity reported as dead code.

Differential Revision: https://reviews.llvm.org/D108210