Introduce ABS as HIR nodes.

NOTE: step 1 of 2 for
"Introduce MIN/MAX/ABS as HIR nodes."

Rationale:
Having explicit MIN/MAX/ABS operations (in contrast
with intrinsics) simplifies recognition and optimization
of these common operations (e.g. constant folding, hoisting,
detection of saturation arithmetic). Furthermore, mapping
conditionals, selectors, intrinsics, etc. (some still TBD)
onto these operations generalizes the way they are optimized
downstream substantially.

Bug: b/65164101

Test: test-art-host,target
Change-Id: I9c93987197216158ba02c8aca2385086adedabc4