[llvm] [X86] IsElementEquivalent - add handling for ISD::BITCASTS from smaller vector elements (PR #139741)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

apart from the movsd issue on 32-bit targets (which I'm not sure what to do tbh), I've now addressed the remaining regressions

https://github.com/llvm/llvm-project/pull/139741