[X86] Add an isel pattern for (i8 (trunc (i16 (bitconvert (v16i1 X))))) to...

[X86] Add an isel pattern for (i8 (trunc (i16 (bitconvert (v16i1 X))))) to avoid an extra EXTRACT_SUBREG

Since we can only copy to GR32 we had to EXTRACT from GR32, but
we would first go to GR16 and then the truncate would extra again
to GR8. This adds a special case to go directly from GR32 to GR8.
This would eventually get cleaned up, but though maybe we should
avoid doing it in the first place. Our k-register handling is weird
and we could probably stand to have some more special ISD nodes
for the conversions so the i32 type would be explicit.