[InstCombine] Fix use after free when removing unreachable code (PR64235)

In degenerate cases, it is possible for unreachable code removal
to remove the current instruction. However, we still return the
instruction to report a change, resulting in a use after free.

Instead, perform the change reporting in the same way as
eraseInstFromFunction() does, by directly setting MadeIRChange
and returning nullptr.

Fixes https://github.com/llvm/llvm-project/issues/64235.