[InstCombine] remove casts from splat-a-bit pattern

https://alive2.llvm.org/ce/z/_AivbM

This case seems clear since we can reduce instruction count
and avoid an intermediate type change, but we might want to
use mask-and-compare for other sequences.

Currently, we can generate more instructions on some related
patterns by trying to use bit-hacks instead of mask+cmp, so
something is not behaving as expected.