This methods returns an RValue<T> type so it's safer than 'loadValue()'.

Bug: b/155302798
Change-Id: Ib48819bd0a1b64ae2d22eda4910ad0b551b1eed7
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44670
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>