[llvm] [AArch64][GlobalISel] Take abs scalar codegen closer to SDAG (PR #84886)

[Thorsten Schütt via llvm-commits]

================
@@ -3914,8 +3914,12 @@ LegalizerHelper::lower(MachineInstr &MI, unsigned TypeIdx, LLT LowerHintTy) {
   case G_SSHLSAT:
   case G_USHLSAT:
     return lowerShlSat(MI);
-  case G_ABS:
+  case G_ABS: {
+    LLT Ty = MRI.getType(MI.getOperand(0).getReg());
+    if (Ty.isScalar() &&  LI.isLegalOrCustom({G_ICMP, {Ty, Ty}}))
+        return lowerAbsToCNeg(MI);
----------------
tschuett wrote:

You probably should check for:
```
isLegalOrCustom({G_ICMP, {Ty, Ty}}) &&
isLegalOrCustom({G_SELECT, {Ty, Ty}}) &&
isLegalOrCustom({G_SUB, {Ty}}) &&
isLegalOrCustom({G_CONSTANT, {Ty}})
```


https://github.com/llvm/llvm-project/pull/84886