[X86] Make EmitCmp into a static function and explicitly return chain result for STRICT_FCMP. NFCI

The only thing its getting from the X86TargetLowering class is
the subtarget which we can easily pass. This function only has
one call site now since this might help the compiler inline it.

Explicitly return both the flag result and the chain result for
STRICT_FCMP nodes. This removes an assumption in the caller that
getValue(1) is the right way to get the chain.