Ensure NI_Vector256_GetLower has the right simdSize and improve codegen slightly

src/coreclr/jit/hwintrinsiccodegenxarch.cpp

@@ -1093,12 +1093,23 @@ void CodeGen::genBaseIntrinsic(GenTreeHWIntrinsic* node)
         {
             if (op1->isContained() || op1->isUsedFromSpillTemp())
             {
-                genHWIntrinsic_R_RM(node, ins, attr, targetReg, op1);
+                // We want to always emit the EA_16BYTE version here.
+                //
+                // For ToVector256Unsafe the upper bits don't matter and for GetLower we
+                // only actually need the lower 16-bytes, so we can just be "more efficient"
+
+                genHWIntrinsic_R_RM(node, ins, EA_16BYTE, targetReg, op1);
             }
             else
             {
+                // We want to always emit the EA_32BYTE version here.
+                //
+                // For ToVector256Unsafe the upper bits don't matter and this allows same
+                // register moves to be elided. For GetLower we're getting a Vector128 and
+                // so the upper bits aren't impactful either allowing the same.
+
                 // Just use movaps for reg->reg moves as it has zero-latency on modern CPUs
-                emit->emitIns_Mov(INS_movaps, attr, targetReg, op1Reg, /* canSkip */ true);
+                emit->emitIns_Mov(INS_movaps, EA_32BYTE, targetReg, op1Reg, /* canSkip */ true);
             }
             break;
         }

src/coreclr/jit/hwintrinsicxarch.cpp

@@ -1318,11 +1318,12 @@ GenTree* Compiler::impBaseIntrinsic(NamedIntrinsic        intrinsic,
                             op1 = gtNewSimdHWIntrinsicNode(simdType, op1, gtNewIconNode(0xD8), NI_AVX2_Permute4x64,
                                                            simdOtherJitType, simdSize);
 
-                            simdSize = 16;
                             simdType = TYP_SIMD16;
 
                             op1 = gtNewSimdHWIntrinsicNode(simdType, op1, NI_Vector256_GetLower, simdBaseJitType,
                                                            simdSize);
+
+                            simdSize = 16;
                         }
                         break;
                     }

src/coreclr/jit/lowerxarch.cpp

@@ -2624,7 +2624,7 @@ void Lowering::LowerHWIntrinsicGetElement(GenTreeHWIntrinsic* node)
             //   ...
             //   op1 = op1.GetLower();
 
-            tmp1 = comp->gtNewSimdHWIntrinsicNode(TYP_SIMD16, op1, NI_Vector256_GetLower, simdBaseJitType, 16);
+            tmp1 = comp->gtNewSimdHWIntrinsicNode(TYP_SIMD16, op1, NI_Vector256_GetLower, simdBaseJitType, simdSize);
             BlockRange().InsertBefore(node, tmp1);
             LowerNode(tmp1);
         }