[llvm] [GlobalISel] add KnownBits tracking for G_UMULH and G_SMULH (PR #158445)

[Matt Arsenault via llvm-commits]

================
@@ -366,6 +366,22 @@ void GISelValueTracking::computeKnownBitsImpl(Register R, KnownBits &Known,
     Known = KnownBits::mul(Known, Known2);
     break;
   }
+  case TargetOpcode::G_UMULH: {
+    computeKnownBitsImpl(MI.getOperand(2).getReg(), Known, DemandedElts,
+                         Depth + 1);
+    computeKnownBitsImpl(MI.getOperand(1).getReg(), Known2, DemandedElts,
----------------
arsenm wrote:

Could early exit if the first one is fully unknown 

https://github.com/llvm/llvm-project/pull/158445