tint: Generalize sem::Vector to allow 16bits subtype

This patch make `sem::Vector::Size()` and `Align()` more general to
prepare for subtypes of 16bits, e.g. f16. Previously these methods
assert that the subtypes are of 4bytes.

Change-Id: Ic939f177e87faabdf17d1d404cee82a91072a179
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/90163
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Zhaoming Jiang <zhaoming.jiang@intel.com>

src/tint/sem/vector.cc

@@ -52,33 +52,17 @@ bool Vector::IsConstructible() const {
 }
 
 uint32_t Vector::Size() const {
-    return SizeOf(width_);
+    return subtype_->Size() * width_;
 }
 
 uint32_t Vector::Align() const {
-    return AlignOf(width_);
-}
-
-uint32_t Vector::SizeOf(uint32_t width) {
-    switch (width) {
+    switch (width_) {
         case 2:
-            return 8;
+            return subtype_->Size() * 2;
         case 3:
-            return 12;
+            return subtype_->Size() * 4;
         case 4:
-            return 16;
-    }
-    return 0;  // Unreachable
-}
-
-uint32_t Vector::AlignOf(uint32_t width) {
-    switch (width) {
-        case 2:
-            return 8;
-        case 3:
-            return 16;
-        case 4:
-            return 16;
+            return subtype_->Size() * 4;
     }
     return 0;  // Unreachable
 }