[X86] Use CMOVNS for abs instead of CMOVGE.

CMOVGE reads SF and OF. CMOVNS only reads SF. This matches with
other recent changes to use a single flag where possible. It also
matches gcc codegen.

I believe this technically changes whether the conditioanl move happens
on INT_MIN, but for INT_MIN both registers are the same so it doesn't
matter.

Differential Revision: https://reviews.llvm.org/D111826