DecomposeStridedMatrix: Avoid reserved keyword

The `mat` token is a reserved keyword in WGSL, so don't try to create
identifiers that using it.

Bug: tint:1438
Change-Id: If3840aff35ab0ea937df776cd95920dd55157d9f
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/80960
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
diff --git a/test/layout/storage/mat2x2/stride/16.spvasm.expected.spvasm b/test/layout/storage/mat2x2/stride/16.spvasm.expected.spvasm
index f5ee801..c003603 100644
--- a/test/layout/storage/mat2x2/stride/16.spvasm.expected.spvasm
+++ b/test/layout/storage/mat2x2/stride/16.spvasm.expected.spvasm
@@ -15,7 +15,7 @@
                OpName %arr_to_mat2x2_stride_16 "arr_to_mat2x2_stride_16"
                OpName %arr "arr"
                OpName %mat2x2_stride_16_to_arr "mat2x2_stride_16_to_arr"
-               OpName %mat "mat"
+               OpName %m "m"
                OpName %f_1 "f_1"
                OpName %f "f"
                OpDecorate %SSBO Block
@@ -52,11 +52,11 @@
                OpReturnValue %21
                OpFunctionEnd
 %mat2x2_stride_16_to_arr = OpFunction %_arr_strided_arr_uint_2 None %22
-        %mat = OpFunctionParameter %mat2v2float
+          %m = OpFunctionParameter %mat2v2float
          %25 = OpLabel
-         %26 = OpCompositeExtract %v2float %mat 0
+         %26 = OpCompositeExtract %v2float %m 0
          %27 = OpCompositeConstruct %strided_arr %26
-         %28 = OpCompositeExtract %v2float %mat 1
+         %28 = OpCompositeExtract %v2float %m 1
          %29 = OpCompositeConstruct %strided_arr %28
          %30 = OpCompositeConstruct %_arr_strided_arr_uint_2 %27 %29
                OpReturnValue %30