i965/fs: Disable opt_sampler_eot for textureGather

i965/fs: Disable opt_sampler_eot for textureGather

The opt_sampler_eot optimisation seems to break when the last
instruction is SHADER_OPCODE_TG4. A bunch of Piglit tests end up doing
this so it causes a lot of regressions. I can't find any documentation
or known workarounds to indicate that this is expected behaviour, but
considering that this is probably a pretty unlikely situation in a
real use case we might as well disable it in order to avoid the
regressions. In total this fixes 451 tests.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>