[x86] add AVX2 cost model entries for maxnum of 256-bit vectors

As noticed in D90554 ,
the AVX2 costs for 256-bit vectors did not include FMAXNUM entries,
so we fell back to AVX1 which assumes those ops will be split into
128-bit halves or something close to that.

Differential Revision: https://reviews.llvm.org/D90613