[GlobalISel] Look through truncs and extends in narrowScalarShift

If a G_SHL is fed by a G_CONSTANT, the lower and upper bits of the source can be
shifted individually by the constant shift amount.

However in case the shift amount came from a G_TRUNC(G_CONSTANT), the generic shift legalization
code was used, producing intermediate shifts that are potentially illegal on some targets.

This change teaches narrowScalarShift to look through G_TRUNCs and G_*EXTs.

Reviewed By: paquette

Differential Revision: https://reviews.llvm.org/D89100
diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index 08d440b..e12b3ed 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -4877,10 +4877,10 @@
   const LLT HalfTy = LLT::scalar(NewBitSize);
   const LLT CondTy = LLT::scalar(1);
 
-  if (const MachineInstr *KShiftAmt =
-          getOpcodeDef(TargetOpcode::G_CONSTANT, Amt, MRI)) {
-    return narrowScalarShiftByConstant(
-        MI, KShiftAmt->getOperand(1).getCImm()->getValue(), HalfTy, ShiftAmtTy);
+  if (auto VRegAndVal =
+          getConstantVRegValWithLookThrough(Amt, MRI, true, false)) {
+    return narrowScalarShiftByConstant(MI, VRegAndVal->Value, HalfTy,
+                                       ShiftAmtTy);
   }
 
   // TODO: Expand with known bits.