tint/resolver: Pre-allocate containers

Reserve the memory for the dependency graph containers.
Reduces expensive heap allocations.

Change-Id: Ia650fcb9f9e461afb8b240c9472a9a478a9968d1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/98081
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/tint/utils/unique_vector.h

@@ -93,6 +93,13 @@ struct UniqueVector {
     /// @returns a const reference to the internal vector
     operator const std::vector<T>&() const { return vector; }
 
+    /// Pre-allocates `count` elements in the vector and set
+    /// @param count the number of elements to pre-allocate
+    void reserve(size_t count) {
+        vector.reserve(count);
+        set.reserve(count);
+    }
+
     /// Removes the last element from the vector
     /// @returns the popped element
     T pop_back() {