This is just a refactoring which avoids storing the type as a Variable
fields, since the 'LValue<T>' subclass knows the type statically and we
only need it during materialization.

This gives us a vtable for overloading other methods in subsequent
changes, at no additional storage cost for Variable.

Bug: b/155302798
Change-Id: I36f6ef8f5f4d2b50ba04d539dc8b9735481c816a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44668
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>