Reduce default vec size in composite.

This CL decreases the default vector size in Composite from 8 to 4.
There will, probably, be a lot more vectors then arrays and structs so
this should reduce the used memory slightly.

Change-Id: I0a47780736192b43189cd211d34744a4f8028b97
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/114200
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>

src/tint/constant/composite.h

@@ -57,7 +57,7 @@ class Composite : public Castable<Composite, constant::Constant> {
     /// The composite type
     type::Type const* const type;
     /// The composite elements
-    const utils::Vector<const constant::Constant*, 8> elements;
+    const utils::Vector<const constant::Constant*, 4> elements;
     /// True if all elements are zero
     const bool all_zero;
     /// True if any element is zero