[llvm] [GlobalISel] Add computeNumSignBits for SHL (PR #152067)

[David Green via llvm-commits]

================
@@ -1892,6 +1892,38 @@ unsigned GISelValueTracking::computeNumSignBits(Register R,
       FirstAnswer = std::min<uint64_t>(FirstAnswer + *C, TyBits);
     break;
   }
+  case TargetOpcode::G_SHL: {
+    Register Src1 = MI.getOperand(1).getReg();
+    Register Src2 = MI.getOperand(2).getReg();
+    if (std::optional<ConstantRange> ShAmtRange =
+            getValidShiftAmountRange(Src2, DemandedElts, Depth + 1)) {
+      uint64_t MaxShAmt = ShAmtRange->getUnsignedMax().getZExtValue();
+      uint64_t MinShAmt = ShAmtRange->getUnsignedMin().getZExtValue();
+
+      MachineInstr &ExtMI = *MRI.getVRegDef(Src1);
+      unsigned ExtOpc = ExtMI.getOpcode();
+
+      if (ExtOpc == TargetOpcode::G_SEXT || ExtOpc == TargetOpcode::G_ZEXT ||
----------------
davemgreen wrote:

The comment would be useful, to keep from SDAG.

https://github.com/llvm/llvm-project/pull/152067