[x86] improve the default expansion of uaddsat/usubsat

This is yet another step towards solving PR14613:
https://bugs.llvm.org/show_bug.cgi?id=14613

uaddsat X, Y --> (X >u (X + Y)) ? -1 : X + Y
usubsat X, Y --> (X >u Y) ? X - Y : 0

We can't count on a sane vector ISA, so override the default (umin/umax)
expansion of unsigned add/sub saturate in cases where we do not have umin/umax.

Differential Revision: https://reviews.llvm.org/D59006

llvm-svn: 356855