i965/fs: Make lower_load_payload etc. appear in INTEL_DEBUG=optimizer.

i965/fs: Make lower_load_payload etc. appear in INTEL_DEBUG=optimizer.

In order to support calling lower_load_payload() inside a condition,
this patch makes OPT() a statement expression:

https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html

We recently did the equivalent change in the vec4 backend (commit
9b8bd67768769b685c25e1276e053505aede5f93).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Acked-by: Jason Ekstrand <jason.ekstrand@intel.com>