[x86] increment/decrement constant vector with min/max in vsetcc lowering (PR39859)

[x86] increment/decrement constant vector with min/max in vsetcc lowering (PR39859)

This is part of fixing PR39859:
https://bugs.llvm.org/show_bug.cgi?id=39859

We have a crippled vector ISA, so we have to invert a typical fold and create min/max here.

As discussed in the bug report, we can probably do better by using saturating subtract when
it's available, but we should have this improvement for the min/max patterns regardless.

Alive proofs:
https://rise4fun.com/Alive/zsf
https://rise4fun.com/Alive/Qrl

Differential Revision: https://reviews.llvm.org/D55515

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349304 91177308-0d34-0410-b5e6-96231b3b80d8