i965: Initialize all member variables of vec4_instruction on construction.

i965: Initialize all member variables of vec4_instruction on construction.

The vec4_instruction object relies on the memory allocator zeroing out
its contents before it's initialized, which is quite an unusual
practice in the C++ world because it ties objects to some specific
allocation scheme, and gives unpredictable results when an object is
created with a different allocator -- Stack allocation, array
allocation, or aggregation inside a different object are some of the
useful possibilities that come to my mind.  Initialize all fields from
the constructor and stop using the zeroing allocator.

Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>