[X86] Fold nested select_cc to select (cmp*ge/le Cond0, Cond1), LHS, Y)

select (cmpeq Cond0, Cond1), LHS, (select (cmpugt Cond0, Cond1), LHS, Y) --> (select (cmpuge Cond0, Cond1), LHS, Y)
etc,

We already perform this fold in DAGCombiner for MVT::i1 comparison results, but these can still appear after legalization (in x86 case with MVT::i8 results), where we need to be more careful about generating new comparison codes.

Pulled out of D101074 to help address the remaining regressions.

Differential Revision: https://reviews.llvm.org/D104707