We have two kinds of "variable" -- those that are directly backed by an
lvalue, and those that are subsets of an indexed location, etc space.

Split them so we don't have to rediscover it later at emit time.

Bug: b/120799499
Change-Id: I7403f76d615d7afa1ee50c014172f07b90262c40
Reviewed-on: https://swiftshader-review.googlesource.com/c/24374Tested-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>