[clang] [Sema] Fix -Wunreachable-code false negative when operands differ only by implicit casts (PR #149972)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

I think this looks correct to me now, but I'd like to leave the final sign-off to @yronglin because they did the original review on #142897

https://github.com/llvm/llvm-project/pull/149972