i965: Remove bad constant buffer constant-reg-already-loaded optimization.

i965: Remove bad constant buffer constant-reg-already-loaded optimization.

Thanks to branching, the state of c->current_const[i].index at the point
of emitting constant loads for this instruction may not match the actual
constant currently loaded in the reg at runtime.  Fixes a regression in my
GLSL program for idr's class since b58b3a786aa38dcc9d72144c2cc691151e46e3d5.