assembler: Don't change the size of opcodes!

assembler: Don't change the size of opcodes!

Until now, the assembler had relocation-related fields added to struct
brw_instruction. This changes the size of the structure and break code
assuming the opcode structure is really 16 bytes, for instance the
emission code in brw_eu_emit.c.

With this commit, we build on the infrastructure that slowly emerged in
the few previous commits to add a relocatable instruction with the
needed fields.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>