Fix: Only skip AND masking for unsigned narrow types in NarrowWithSaturation

The inputsAlreadyClamped optimization was incorrectly applied to all types,
but it's only valid for unsigned narrow types (TYP_UBYTE, TYP_USHORT).

For signed narrow types, the AND masking is still required to clear the
sign-extended upper bits before PackUnsignedSaturate can correctly pack
the values. Without this masking, negative values would have incorrect
upper bits that would cause PackUnsignedSaturate to produce wrong results.

This fixes System.Numerics.Tests.GenericVectorTests.NarrowWithSaturationInt32Test

Author: laveeshb

src/coreclr/jit/hwintrinsicxarch.cpp

@@ -3541,8 +3541,14 @@ GenTree* Compiler::impSpecialIntrinsic(NamedIntrinsic        intrinsic,
                     op2 = gtNewSimdMinMaxNode(retType, op2, gtCloneExpr(maxCns), simdBaseType, simdSize,
                                               /* isMax */ false, /* isMagnitude */ false, /* isNumber */ false);
 
-                    retNode = gtNewSimdNarrowNode(retType, op1, op2, narrowSimdBaseType, simdSize,
-                                                  /* inputsAlreadyClamped */ true);
+                    // For unsigned narrow types (TYP_UBYTE, TYP_USHORT), the clamping already ensures
+                    // the upper bits are zero, so the AND masking in gtNewSimdNarrowNode is redundant.
+                    // For signed narrow types, we still need the AND masking to clear sign-extended bits
+                    // before PackUnsignedSaturate can correctly pack the values.
+                    bool inputsAlreadyClamped = varTypeIsUnsigned(narrowSimdBaseType);
+
+                    retNode =
+                        gtNewSimdNarrowNode(retType, op1, op2, narrowSimdBaseType, simdSize, inputsAlreadyClamped);
                 }
             }
             break;