[llvm] [GlobalISel] Add G_SUB for computeNumSignBits (PR #158384)

[Matt Arsenault via llvm-commits]

================
@@ -1976,6 +1976,42 @@ unsigned GISelValueTracking::computeNumSignBits(Register R,
 
     break;
   }
+  case TargetOpcode::G_SUB: {
+    Register Src1 = MI.getOperand(1).getReg();
+    unsigned Src1NumSignBits =
+        computeNumSignBits(Src1, DemandedElts, Depth + 1);
+    if (Src1NumSignBits == 1)
+      return 1; // Early Out.
----------------
arsenm wrote:

This doesn't match the DAG; the first check should be on operand 2, not 1

https://github.com/llvm/llvm-project/pull/158384