test: Add tests to verify variables are zero initialized

Bug: tint:938
Fixed: tint:759
Change-Id: I5794d7f40ec154c55240b163316ed45c7f14b190
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/56547
Reviewed-by: James Price <jrprice@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/var/initialization/function/struct.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 %main "main"
+               OpExecutionMode %main LocalSize 1 1 1
+               OpName %main "main"
+               OpName %S "S"
+               OpMemberName %S 0 "a"
+               OpMemberName %S 1 "b"
+               OpName %v "v"
+               OpMemberDecorate %S 0 Offset 0
+               OpMemberDecorate %S 1 Offset 4
+       %void = OpTypeVoid
+          %1 = OpTypeFunction %void
+        %int = OpTypeInt 32 1
+      %float = OpTypeFloat 32
+          %S = OpTypeStruct %int %float
+%_ptr_Function_S = OpTypePointer Function %S
+         %10 = OpConstantNull %S
+       %main = OpFunction %void None %1
+          %4 = OpLabel
+          %v = OpVariable %_ptr_Function_S Function %10
+         %12 = OpLoad %S %v
+               OpReturn
+               OpFunctionEnd