test: Add test cases for zero value constructors

Fixed: tint:477
Change-Id: I087c24904ab2f38524ab2c39092fbfb277fdba3b
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/55256
Auto-Submit: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

test/expressions/zero_init/mat4x2/f32.wgsl

@@ -0,0 +1,3 @@
+fn f() {
+    var v = mat4x2<f32>();
+}

test/expressions/zero_init/mat4x2/f32.wgsl.expected.hlsl

@@ -0,0 +1,8 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  float4x2 v = float4x2(0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f);
+}

test/expressions/zero_init/mat4x2/f32.wgsl.expected.msl

@@ -0,0 +1,7 @@
+#include <metal_stdlib>
+
+using namespace metal;
+void f() {
+  float4x2 v = float4x2();
+}
+

test/expressions/zero_init/mat4x2/f32.wgsl.expected.spvasm

@@ -0,0 +1,29 @@
+; SPIR-V
+; Version: 1.3
+; Generator: Google Tint Compiler; 0
+; Bound: 13
+; Schema: 0
+               OpCapability Shader
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint GLCompute %unused_entry_point "unused_entry_point"
+               OpExecutionMode %unused_entry_point LocalSize 1 1 1
+               OpName %unused_entry_point "unused_entry_point"
+               OpName %f "f"
+               OpName %v "v"
+       %void = OpTypeVoid
+          %1 = OpTypeFunction %void
+      %float = OpTypeFloat 32
+    %v2float = OpTypeVector %float 2
+%mat4v2float = OpTypeMatrix %v2float 4
+         %10 = OpConstantNull %mat4v2float
+%_ptr_Function_mat4v2float = OpTypePointer Function %mat4v2float
+%unused_entry_point = OpFunction %void None %1
+          %4 = OpLabel
+               OpReturn
+               OpFunctionEnd
+          %f = OpFunction %void None %1
+          %6 = OpLabel
+          %v = OpVariable %_ptr_Function_mat4v2float Function %10
+               OpStore %v %10
+               OpReturn
+               OpFunctionEnd

test/expressions/zero_init/mat4x2/f32.wgsl.expected.wgsl

@@ -0,0 +1,3 @@
+fn f() {
+  var v = mat4x2<f32>();
+}