[x86] adjust cost model values for minnum/maxnum with fast-math-flags

Without FMF, we lower these intrinsics into something like this:

vmaxsd	%xmm0, %xmm1, %xmm2
vcmpunordsd	%xmm0, %xmm0, %xmm0
vblendvpd	%xmm0, %xmm1, %xmm2, %xmm0

But if we can ignore NANs, the single min/max instruction is enough
because there is no need to fix up the x86 logic that corresponds to
X > Y ? X : Y.

We probably want to make other adjustments for FP intrinsics with FMF
to account for specialized codegen (for example, FSQRT).

Differential Revision: https://reviews.llvm.org/D92337