SpirvShader: Make value names more consistent

SpirvShader: Make value names more consistent

Rename Object::Kind::Value to Object::Kind::Intermediate.
Rename SpirvRoutine::Value to SpirvRoutine::Variable.

Change-Id: I1dba30687cbf979d80b0deeca7a96e64a88c94db
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28393
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>