i965/vec4: Fix broken IR annotation in debug output.

Commit 70953b5 (i965: Initialize all member variables of
vec4_instruction on construction) inadvertently added a line to the
vec4_instruction constructor setting this->ir to NULL, wiping out the
previously set value.  As a result, ever since then, the output of
INTEL_DEBUG=vs and INTEL_DEBUG=gs has been missing IR annotations.
Experiment: more tree grafting
Reenable back end optimizations
Debug printfs