Just like the non-constant array index lowering pass, compare all N
indices at once. For accesses to a vec4, this saves 3 comparison
instructions on a vector architecture.
Reviewed-by: Eric Anholt <eric@anholt.net>
If the non-constant index was in the LHS of an assignment, any
existing condititon on that assignment would be lost.
Reviewed-by: Eric Anholt <eric@anholt.net>