vc4: Avoid the save/restore of r3 for raddr conflicts, just use ra31.

vc4: Avoid the save/restore of r3 for raddr conflicts, just use ra31.

Turns out this was harmful in code quality:

total instructions in shared programs: 39487 -> 38845 (-1.63%)
instructions in affected programs:     22522 -> 21880 (-2.85%)

This costs us yet another register, which is painful since it means more
programs might fail to compile).  However, the alternative was causing us
trouble where we'd save/restore r3 while it contained a MIN-ed direct
texture offset, causing the kernel to fail to validate our shaders (such
as in GLB2.7).