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

This is an enhancement to D77895 to avoid another
round-trip from XMM->GPR->XMM. This time we handle
the case of starting/ending with an f64 and casting
to signed i32 as the intermediate value.

It's a bit more involved than I initially assumed
because we need to use target-specific opcodes to
represent the non-standard cast ops.

Differential Revision: https://reviews.llvm.org/D78362