st/mesa: fix broken translation of negative register indexes

st/mesa: fix broken translation of negative register indexes

A src register's index can be negative if we're doing indirect
addressing into the constant buffer.  Ex: MOV OUT[1], CONST[ADDR[0].x-3]

This fixes the piglit vp-arl-neg-array.vpfp test.

Before this change we were going out of bounds of the t->constants[]
array and getting garbage that later triggered an assertion.