This is not a template class, so its methods don't have to be inlined.
Nor are they trivial enough to save us anything in performance or
binary size if they were inlined. Instead, this could reduce binary
size a bit because there are a large number of rr::Variable uses.

Bug: chromium:1086501
Change-Id: I1a6d078628ed7e634e0d90d6d8455e7d2938d134
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/45388
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>