[wasm-simd] Fix i64x2 splat isel on ia32

In the non-avx case, we are incorrectly using DefineSameAsFirst, which
is wrong, since operand0 is a 32-bit register, but the output should be
a simd register. This error was caught by running the test with flag:
--no-enable-avx, leading to a register allocator verifier failure.

Drive by cleanup to remove duplicate cases for avx and see in i64x2 replace
lane.

Bug: v8:9728
Change-Id: I96490c82e13660fc41e72ac523e5be37b051b624
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1866963
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64363}

src/compiler/backend/ia32/instruction-selector-ia32.cc

@@ -2089,11 +2089,7 @@ void InstructionSelector::VisitI64x2SplatI32Pair(Node* node) {
   IA32OperandGenerator g(this);
   InstructionOperand operand0 = g.UseRegister(node->InputAt(0));
   InstructionOperand operand1 = g.Use(node->InputAt(1));
-  if (IsSupported(AVX)) {
   Emit(kIA32I64x2SplatI32Pair, g.DefineAsRegister(node), operand0, operand1);
-  } else {
-    Emit(kIA32I64x2SplatI32Pair, g.DefineSameAsFirst(node), operand0, operand1);
-  }
 }
 
 void InstructionSelector::VisitI64x2ReplaceLaneI32Pair(Node* node) {
@@ -2102,13 +2098,8 @@ void InstructionSelector::VisitI64x2ReplaceLaneI32Pair(Node* node) {
   InstructionOperand lane = g.UseImmediate(OpParameter<int32_t>(node->op()));
   InstructionOperand low = g.Use(node->InputAt(1));
   InstructionOperand high = g.Use(node->InputAt(2));
-  if (IsSupported(AVX)) {
   Emit(kIA32I64x2ReplaceLaneI32Pair, g.DefineSameAsFirst(node), operand, lane,
        low, high);
-  } else {
-    Emit(kIA32I64x2ReplaceLaneI32Pair, g.DefineSameAsFirst(node), operand, lane,
-         low, high);
-  }
 }
 
 void InstructionSelector::VisitF32x4Splat(Node* node) {