[x86] use vector instructions to lower FP->int->FP casts

As discussed in PR36617:
https://bugs.llvm.org/show_bug.cgi?id=36617#c13
...we can avoid the likely slow round-trip from XMM to GPR to XMM
by using the vector versions of the convert instructions.

Based on experimental results from recent Intel/AMD chips, we don't
need to worry about triggering denorm stalls while operating on
garbage data in the high lanes with convert instructions, so this is
expected to always be as good or better perf than the scalar
instruction equivalent. FP exceptions are also not a concern because
strict code should not be using the regular SDAG opcodes.

Differential Revision: https://reviews.llvm.org/D77895