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
diff --git a/compiler/optimizing/scheduler.cc b/compiler/optimizing/scheduler.cc
index bb28d50..cdbe483 100644
--- a/compiler/optimizing/scheduler.cc
+++ b/compiler/optimizing/scheduler.cc
@@ -667,7 +667,8 @@
   // HUnaryOperation (or HBinaryOperation), check in debug mode that we have
   // the exhaustive lists here.
   if (instruction->IsUnaryOperation()) {
-    DCHECK(instruction->IsBooleanNot() ||
+    DCHECK(instruction->IsAbs() ||
+           instruction->IsBooleanNot() ||
            instruction->IsNot() ||
            instruction->IsNeg()) << "unexpected instruction " << instruction->DebugName();
     return true;