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>

assembler/brw_structs.h

@@ -1463,9 +1463,6 @@ struct brw_instruction
       GLuint ud;
       float f;
    } bits3;
-
-   char *first_reloc_target, *second_reloc_target; // first for JIP, second for UIP
-   GLint first_reloc_offset, second_reloc_offset; // in number of instructions
 };
 
 struct brw_compact_instruction {