spirv-reader: switch to HLSL-style pipeline IO

- When storing to sample_mask output, write to the 0th element
- Only make a return struct if it has members
- Adjust type signedness coercion when loading special builtins.
- Adapt tests

- Update expectations for end-to-end tests

- Handle sample_mask with stride
  Input variables normally don't have layout. But they can have it
  up through SPIR-V 1.4.
  Handle this case in the SPIR-V reader, by seeing through the
  intermediate alias type created for the strided array type.

Bug: tint:508
Change-Id: I0f19dc1305d3f250dbbc0698a602288c34245274
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/54743
Auto-Submit: David Neto <dneto@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/ptr_ref/store/local/struct_field.spvasm.expected.wgsl

@@ -2,9 +2,13 @@ struct S {
   i : i32;
 };
 
-[[stage(compute)]]
-fn main() {
+fn main_1() {
   var V : S;
   V.i = 5;
   return;
 }
+
+[[stage(compute)]]
+fn main() {
+  main_1();
+}