test: Add some basic vector / matrix access tests

Change-Id: I774d7e4b6fdf9d1507bd9cbf3be00bd4113e8162
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51222
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/test/access/let/matrix.spvasm.expected.spvasm b/test/access/let/matrix.spvasm.expected.spvasm
new file mode 100644
index 0000000..fac85ae
--- /dev/null
+++ b/test/access/let/matrix.spvasm.expected.spvasm
@@ -0,0 +1,36 @@
+; SPIR-V
+; Version: 1.3
+; Generator: Google Tint Compiler; 0
+; Bound: 25
+; Schema: 0
+               OpCapability Shader
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint GLCompute %main "main"
+               OpExecutionMode %main LocalSize 1 1 1
+               OpName %main "main"
+       %void = OpTypeVoid
+          %1 = OpTypeFunction %void
+      %float = OpTypeFloat 32
+    %v3float = OpTypeVector %float 3
+%mat3v3float = OpTypeMatrix %v3float 3
+    %float_1 = OpConstant %float 1
+    %float_2 = OpConstant %float 2
+    %float_3 = OpConstant %float 3
+         %11 = OpConstantComposite %v3float %float_1 %float_2 %float_3
+    %float_4 = OpConstant %float 4
+    %float_5 = OpConstant %float 5
+    %float_6 = OpConstant %float 6
+         %15 = OpConstantComposite %v3float %float_4 %float_5 %float_6
+    %float_7 = OpConstant %float 7
+    %float_8 = OpConstant %float 8
+    %float_9 = OpConstant %float 9
+         %19 = OpConstantComposite %v3float %float_7 %float_8 %float_9
+         %20 = OpConstantComposite %mat3v3float %11 %15 %19
+       %uint = OpTypeInt 32 0
+     %uint_1 = OpConstant %uint 1
+       %main = OpFunction %void None %1
+          %4 = OpLabel
+         %23 = OpCompositeExtract %v3float %20 1
+         %24 = OpCompositeExtract %float %23 1
+               OpReturn
+               OpFunctionEnd