ityp_array: Allow using a T whose underlying type is signed.

When trying to use ityp_array with an enum class whose underlying
type is int, warnings were fired because of a comparison between signed
and unsigned integers. Fix this by explicitly casting Size to `I` using
a constructor cast.

Bug: dawn:635
Change-Id: I5ee0101684e5847ec5ec6f71a9657fcce839a2a5
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/38106
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Stephen White <senorblanco@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

src/common/ityp_array.h

@@ -44,25 +44,25 @@ namespace ityp {
 
         Value& operator[](Index i) {
             I index = static_cast<I>(i);
-            ASSERT(index >= 0 && index < Size);
+            ASSERT(index >= 0 && index < I(Size));
             return Base::operator[](index);
         }
 
         constexpr const Value& operator[](Index i) const {
             I index = static_cast<I>(i);
-            ASSERT(index >= 0 && index < Size);
+            ASSERT(index >= 0 && index < I(Size));
             return Base::operator[](index);
         }
 
         Value& at(Index i) {
             I index = static_cast<I>(i);
-            ASSERT(index >= 0 && index < Size);
+            ASSERT(index >= 0 && index < I(Size));
             return Base::at(index);
         }
 
         constexpr const Value& at(Index i) const {
             I index = static_cast<I>(i);
-            ASSERT(index >= 0 && index < Size);
+            ASSERT(index >= 0 && index < I(Size));
             return Base::at(index);
         }
 
@@ -83,7 +83,7 @@ namespace ityp {
         }
 
         constexpr Index size() const {
-            return Index(static_cast<I>(Size));
+            return Index(I(Size));
         }
 
         using Base::back;