i965/vec4: Don't overwrite op[1] when doing a UBO load.

i965/vec4: Don't overwrite op[1] when doing a UBO load.

Prior to the GLSL CSE pass, all of our testing happened to have a freshly
computed temporary in op[1], from the multiply by 16 to get a byte offset.
As of CSE you'll get var_refs of a reused value when you've got multiple
loads from the same offset.

Make a proper temporary for computing our temporary value, to avoid
shifting the value farther and farther down.  Avoids a regression in
gs-float-array-variable-index

Reviewed-by: Paul Berry <stereotype441@gmail.com>