[InstCombine] fold icmp of truncated left shift, part 2

(trunc (1 << Y) to iN) == 2**C --> Y == C
(trunc (1 << Y) to iN) != 2**C --> Y != C
https://alive2.llvm.org/ce/z/xnFPo5

Follow-up to d9e1f9d7. This was a suggested
enhancement mentioned in issue #51889.