i965: add assert to while_jumps_before_offset()
i965: add assert to while_jumps_before_offset()

jip should always be negative here as its the result of
do instruction - while instruction.

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler