[spirv-writer] Simplify the texture intrinsic tests

This CL removes the complete coverage of the possible input formats
infavor of simpler testing of a few of the inputs. This should be
sufficient coverage and makes the tests easier to read.

The incorrect OpConstantNull for the samplers and textures has also been
removed.

Change-Id: I9cd7d6299f0454bb8e2019968e0422221e4ec183
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/28581
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: dan sinclair <dsinclair@chromium.org>

src/writer/spirv/builder.cc

@@ -597,7 +597,7 @@ bool Builder::GenerateGlobalVariable(ast::Variable* var) {
                      Operand::Int(ConvertStorageClass(sc))};
   if (var->has_constructor()) {
     ops.push_back(Operand::Int(init_id));
-  } else {
+  } else if (!var->type()->IsTexture() && !var->type()->IsSampler()) {
     // If we don't have a constructor and we're an Output or Private variable
     // then WGSL requires an initializer.
     if (var->storage_class() == ast::StorageClass::kPrivate ||