[InstCombine] add narrowing transform for low-masked binop with zext operand (2nd try)

The 1st try ( afa192cf ) was reverted because it could
cause an infinite loop with constant expressions.

A test for that and an extra condition to enable the transform
are added now. I also added code comments to better describe
the transform and the existing, related transform.

Original commit message:
https://alive2.llvm.org/ce/z/hRy3rE

As shown in D123408, we can produce this pattern when moving
casts around, and we already have a related fold for a binop
with a constant operand.