Contains a base address, per-lane offsets and a boolean to say whether it is uniform across lanes.

Use this instead of the std::pair for GetPointerToData and WalkExplicitLayoutAccessChain.

Provides readability improvements.

Bug: b/126126820
Change-Id: If3a010d7dc37c4f07ca489ae58195cdfed7ea559
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28568Tested-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>