ir_to_mesa: Add support for array dereferences on the LHS of assignments.

ir_to_mesa: Add support for array dereferences on the LHS of assignments.

The big change is to delay address reg setup until the instruction
that needs the deref.  It was hard to use the deref chain support for
the LHS because it does the copy of the dereffed value to a temporary
(to avoid problems when two src regs are array derefs), so we wouldn't
haev a pointer to actual storage in the end.

Fixes glsl-vs-arrays on swrast.