[llvm] [X86] Convert FP compares split by a nested logic op to FP logic (PR #226222)

[Tim Besard via llvm-commits]

maleadt wrote:

OK, restructured a little. There's now a single helper that checks whether an operand can be computed in a vXi1 vector, either because it's a scalar FP compare or because it's element 0 of an already-converted sequence. Any two such operands get combined the same way, so the old SETCC pair code and the separate extension path are gone. The nested case just retries that with each operand of the inner op. The bitcast path is back to its original form, and the extract now uses getExtractVectorElt. The only test changes are operand order.

https://github.com/llvm/llvm-project/pull/226222