[X86] Add DAG combine for negation of CMOV absolute value pattern.

This patch detects the absolute value pattern on the RHS of a
subtract. If we find it we swap the CMOV true/false values and
replace the subtract with an ADD.

There may be a more generic way to do this, but I'm not sure.
Targets that don't have legal or custom ISD::ABS use a generic
expand in DAG combiner already when it sees (neg (abs(x))). I
haven't checked what happens if the neg is a more general subtract.

Fixes PR50991 for X86.

Reviewed By: RKSimon, spatel

Differential Revision: https://reviews.llvm.org/D111858