i965/vec4: Assign correct destination offset to rewritten instruction in register...

i965/vec4: Assign correct destination offset to rewritten instruction in register coalesce.

Because the pass already checks that the destination offset of each
'scan_inst' that needs to be rewritten matches 'inst->src[0].offset'
exactly, the final offset of the rewritten instruction is just the
original destination offset of the copy.  This is in preparation for
adding support for sub-GRF offsets to the VEC4 IR.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>