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/load/global/struct_field.spvasm.expected.spvasm

@@ -1,7 +1,7 @@
 ; SPIR-V
 ; Version: 1.3
 ; Generator: Google Tint Compiler; 0
-; Bound: 18
+; Bound: 21
 ; Schema: 0
                OpCapability Shader
                OpMemoryModel Logical GLSL450
@@ -10,8 +10,9 @@
                OpName %S "S"
                OpMemberName %S 0 "i"
                OpName %V "V"
-               OpName %main "main"
+               OpName %main_1 "main_1"
                OpName %i "i"
+               OpName %main "main"
                OpMemberDecorate %S 0 Offset 0
         %int = OpTypeInt 32 1
           %S = OpTypeStruct %int
@@ -25,7 +26,7 @@
        %uint = OpTypeInt 32 0
      %uint_0 = OpConstant %uint 0
 %_ptr_Private_int = OpTypePointer Private %int
-       %main = OpFunction %void None %6
+     %main_1 = OpFunction %void None %6
           %9 = OpLabel
           %i = OpVariable %_ptr_Function_int Function %12
          %16 = OpAccessChain %_ptr_Private_int %V %uint_0
@@ -33,3 +34,8 @@
                OpStore %i %17
                OpReturn
                OpFunctionEnd
+       %main = OpFunction %void None %6
+         %19 = OpLabel
+         %20 = OpFunctionCall %void %main_1
+               OpReturn
+               OpFunctionEnd