X86: Turn fp selects into mask operations.

double test(double a, double b, double c, double d) { return a<b ? c : d; }

before:
_test:
	ucomisd	%xmm0, %xmm1
	ja	LBB0_2
	movaps	%xmm3, %xmm2
LBB0_2:
	movaps	%xmm2, %xmm0

after:
_test:
	cmpltsd	%xmm1, %xmm0
	andpd	%xmm0, %xmm2
	andnpd	%xmm3, %xmm0
	orpd	%xmm2, %xmm0

Small speedup on Benchmarks/SmallPT