i965: Mark error annotation on correct SIMD16 inst.

i965: Mark error annotation on correct SIMD16 inst.

inst, whose assignment can be seen in the last line of context pointed
to the correct instruction in the SIMD16 program, but src_offset was the
offset from the beginning of the SIMD16 program.

So if an instruction at offset 0x100 in the SIMD16 program was illegal,
we would mark an error on the instruction at offset 0x100 (which is
likely in the SIMD8 program).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>