Validate scalar constructor and implement conversion to vecN<bool> in spir-v backend

After implementing validation and fairly exhaustive tests, discovered
that conversion of scalar vector to bool vector did not work in the
spir-v backend. For module scope variables, we use and rely on the
FoldConstants transform to ensure no conversion needs to take place.
This is necessary because we cannot easily introduce temporary values
and refer to them when casting at module scope. Note that for the same
reason, module-level conversions are always constant foldable, so this
works. For function-level conversions, implemented support to emit a
comparison against a zero value, and store the result in the bool
vector.

Bug: tint:865
Change-Id: I0528045e803f176e03428bc7eac31ae06920bbd7
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/54744
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/transform/spirv.cc

@@ -27,6 +27,7 @@
 #include "src/sem/struct.h"
 #include "src/sem/variable.h"
 #include "src/transform/external_texture_transform.h"
+#include "src/transform/fold_constants.h"
 #include "src/transform/inline_pointer_lets.h"
 #include "src/transform/manager.h"
 #include "src/transform/simplify.h"
@@ -43,6 +44,7 @@ Output Spirv::Run(const Program* in, const DataMap& data) {
   Manager manager;
   manager.Add<InlinePointerLets>();  // Required for arrayLength()
   manager.Add<Simplify>();           // Required for arrayLength()
+  manager.Add<FoldConstants>();
   manager.Add<ExternalTextureTransform>();
   auto transformedInput = manager.Run(in, data);