writer/hlsl: Simplify emission logic, clean up output

And fix issues where global variables would not be emitted unless they were transitively referenced by an entry point.

This change requires crbug.com/tint/697 to be fixed before landing.

Change-Id: I712bd9d369e08c9a3cdfb0f114c3609584f91f28
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/54241
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: James Price <jrprice@google.com>

test/ptr_ref/load/param/ptr.spvasm.expected.hlsl

@@ -11,4 +11,3 @@ void main() {
   const int x_18 = func(x_19, i);
   return;
 }
-

test/ptr_ref/load/param/ptr.wgsl.expected.hlsl

@@ -8,4 +8,3 @@ void main() {
   const int r = func(i, i);
   return;
 }
-