[InstCombine] remove zext-of-icmp fold that may conflict with other folds

This bit-hack transform would cause the new test to infinite loop
after 21d3871b.

The deleted transform has existed for a very long time,
but the profitable parts appear to be handled by other
folds now. This fold could replace 2 instructions with
4 instructions, so it was always in danger of going
overboard.

No tests regress by removing the whole thing.