[X86][AVX] combineBitcastvxi1 - improve handling of vectors truncated to vXi1

If we're truncating to vXi1 from a wider type, then prefer the original wider vector as is simplifies folding the separate truncations + extensions.

AVX1 this is only worth it for v8i1 cases, not v4i1 where we're always better off truncating down to v4i32 for movmsk.

Helps with some regressions encountered in D96609