[llvm] [X86] Remove LowerFCanonicalize and use generic expansion (PR #147730)

[via llvm-commits]

woruyu wrote:

It turns out this is more complex than I initially expected — especially around the handling of f16 in X86/canonicalize-vars-f16-type.ll and ensuring the behavior remains correct across all targets. I'm going to close this patch for now and revisit it once I’ve had more time to think through the design and edge case

https://github.com/llvm/llvm-project/pull/147730