[llvm] [DAG] Replace getValid*ShiftAmountConstant helpers with getValid*ShiftAmount helpers to support KnownBits analysis (PR #93182)

[Simon Pilgrim via llvm-commits]

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@@ -3020,91 +3020,111 @@ SelectionDAG::getValidShiftAmountConstant(SDValue V,
     // Shifting more than the bitwidth is not valid.
     const APInt &ShAmt = SA->getAPIntValue();
     if (ShAmt.ult(BitWidth))
-      return &ShAmt;
+      return ShAmt.getZExtValue();
+  } else {
+    KnownBits KnownAmt =
+        computeKnownBits(V.getOperand(1), DemandedElts, Depth + 1);
+    if (KnownAmt.isConstant() && KnownAmt.getConstant().ult(BitWidth))
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RKSimon wrote:

The problem we're hitting is that in many cases its not just a bitcast of a constant, its often more complex than that and when trying to use value tracking during legalization we only get one chance, we can't wait for combines to simplify things.

https://github.com/llvm/llvm-project/pull/93182