We can use the same register spilling infrastructure for our loads/stores
of indirect access of temp variables, instead of doing an if ladder.

Cuts 50% of instructions and max-temps from 2 KSP shaders in shader-db.
Also causes several other KSP shaders with large bodies and large loop
counts to not be force-unrolled.

The change was originally motivated by NOLTIS slightly modifying register
pressure in piglit temp mat4 array read/write tests, triggering register
allocation failures.