These methods operate on RValue<T> types so they're safer than
'loadValue()' and 'storeValue()'. They also enable overloading
the latter in subsequent changes.

Bug: b/155302798
Change-Id: I30903fc70e353dfd668873122ad7d8f74e857420
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44669
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>