[X86] Prevent LowerSELECT from causing suboptimal codegen for __builtin_ffs(X) - 1.

LowerSELECT sees the CMP with 0 and wants to use a trick with SUB
and SBB. But we can use the flags from the BSF/TZCNT.

Fixes PR46203.

Differential Revision: https://reviews.llvm.org/D81312