Remove redundant sign/zero extension for SIMD broadcasts (#108824)

Author: EgorBo

src/coreclr/jit/lowerxarch.cpp

@@ -10377,6 +10377,22 @@ void Lowering::ContainCheckHWIntrinsic(GenTreeHWIntrinsic* node)
 
                                 op1 = node->Op(1);
                                 op1->ClearContained();
+
+                                // Drop GT_CAST if it doesn't change the op1's bits we're about to broadcast
+                                if (op1->OperIs(GT_CAST) && !op1->gtOverflow() && comp->opts.Tier0OptimizationEnabled())
+                                {
+                                    GenTreeCast* cast = op1->AsCast();
+                                    if (!varTypeIsFloating(cast->CastToType()) &&
+                                        !varTypeIsFloating(cast->CastFromType()) &&
+                                        (genTypeSize(cast->CastToType()) >= genTypeSize(simdBaseType)) &&
+                                        (genTypeSize(cast->CastFromType()) >= genTypeSize(simdBaseType)))
+                                    {
+                                        BlockRange().Remove(op1);
+                                        op1 = cast->CastOp();
+                                        op1->ClearContained();
+                                        node->Op(1) = op1;
+                                    }
+                                }
                             }
                         }
                         break;