This does automatic widening to per-lane so callers don't have to
scatter "is constant?" checks everywhere. This is mostly the right
thing to do -- loads and stores will want to continue specializing
on whether values and offsets are uniform across lanes, but most
other things don't care.

Bug: b/126475489
Change-Id: I1b8693a75e93b00a7972ec06777fbaaa599d2a52
Reviewed-on: https://swiftshader-review.googlesource.com/c/25668Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>