v3d: Use the new lower_to_scratch implementation for indirects on temps.
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.
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