[InstCombine] recode icmp fold in a vector-friendly way; NFC

The transform in question:
icmp (and (trunc W), C2), C1 -> icmp (and W, C2'), C1'

...is still not enabled for vectors, thus no functional change intended.
It's not clear to me if this is a good transform for vectors or even
scalars in general. Changing that behavior may be a follow-on patch.

llvm-svn: 280627