[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
llvm-svn: 349304