[X86] Canonicalize the pattern for __builtin_ffs in a similar way to '__builtin_ffs + 5'

We now emit a move of -1 before the cmov and do the addition after the cmov just like the case with an extra addition.

This may be slightly worse for code size, but is more consistent with other compilers. And we might be able to hoist the mov -1 outside of loops.

llvm-svn: 338613