[x86] avoid flipping sign bits for vector icmp by using known bits

If we know that both operands of an unsigned integer vector comparison are non-negative, 
then it's safe to directly use a signed-compare-greater-than instruction (the only non-equality
integer vector compare predicate provided by SSE/AVX).

We're intentionally not changing the condition code to signed in order to preserve the
existing transforms that use min/max/psubus below here.

This should solve PR33276:
https://bugs.llvm.org/show_bug.cgi?id=33276

Differential Revision: https://reviews.llvm.org/D33862

llvm-svn: 304909