[spirv-writer] Support optional trailing return.

This CL updates the SPIRV-Writer to inject an OpReturn as the trailing
statement in a function if the function does not end with a `discard` or
a `return` statement.

R=bclayton@google.com, dneto@google.com

Fixes: tint:302
Change-Id: I2e7c7beff15ad30c779c591bb75cf97fc0960bf7
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/33160
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Auto-Submit: dan sinclair <dsinclair@chromium.org>

test/compute_boids.wgsl

@@ -26,7 +26,6 @@ fn vert_main() -> void {
       (a_pos.x * cos(angle)) - (a_pos.y * sin(angle)),
       (a_pos.x * sin(angle)) + (a_pos.y * cos(angle)));
   gl_Position = vec4<f32>(pos + a_particlePos, 0.0, 1.0);
-  return;
 }
 
 # fragment shader
@@ -35,7 +34,6 @@ fn vert_main() -> void {
 [[stage(fragment)]]
 fn frag_main() -> void {
   fragColor = vec4<f32>(1.0, 1.0, 1.0, 1.0);
-  return;
 }
 
 # compute shader
@@ -137,6 +135,4 @@ fn comp_main() -> void {
   # Write back
   particlesB.particles[index].pos = vPos;
   particlesB.particles[index].vel = vVel;
-
-  return;
 }

test/cube.wgsl

@@ -28,7 +28,6 @@
 fn vtx_main() -> void {
    Position = uniforms.modelViewProjectionMatrix * cur_position;
    vtxFragColor = color;
-   return;
 }
 
 # Fragment shader
@@ -38,5 +37,4 @@ fn vtx_main() -> void {
 [[stage(fragment)]]
 fn frag_main() -> void {
   outColor = fragColor;
-  return;
 }

test/function.wgsl

@@ -19,5 +19,4 @@ fn main() -> f32 {
 [[stage(compute)]]
 [[workgroup_size(2)]]
 fn ep() -> void {
-  return;
 }

test/simple.wgsl

@@ -15,7 +15,6 @@
 [[location(0)]] var<out> gl_FragColor : vec4<f32>;
 
 fn bar() -> void {
-  return;
 }
 
 [[stage(fragment)]]
@@ -23,5 +22,4 @@ fn main() -> void {
     var a : vec2<f32> = vec2<f32>();
     gl_FragColor = vec4<f32>(0.4, 0.4, 0.8, 1.0);
     bar();
-    return;
 }

test/triangle.wgsl

@@ -24,7 +24,6 @@ const pos : array<vec2<f32>, 3> = array<vec2<f32>, 3>(
 [[stage(vertex)]]
 fn vtx_main() -> void {
   Position = vec4<f32>(pos[VertexIndex], 0.0, 1.0);
-  return;
 }
 
 # Fragment shader
@@ -33,5 +32,4 @@ fn vtx_main() -> void {
 [[stage(fragment)]]
 fn frag_main() -> void {
   outColor = vec4<f32>(1.0, 0.0, 0.0, 1.0);
-  return;
 }