[InstCombine] Verify CmpInst is equality in `foldICmpPow2Test`; PR63327

When D152728 hoisted the code to a helper function, it moved the call
to the helper outside of `foldICmpEquality`, so an equality check is
needed in the helper.

Reviewed By: nikic, fhahn

Differential Revision: https://reviews.llvm.org/D153041