iris: Push heavy memchecker code to DEBUG

iris: Push heavy memchecker code to DEBUG

Invoking VALGRIND_CHECK_MEM_IS_DEFINED pulls in enough code to convince
gcc to not inline __gen_uint and results in a lot of packing code ending
up out-of-line with lots of stack copying. To ameliorate this, only
insert the check inside the packer if DEBUG is defined and instead
perform the validation checking before submitting the batch to the
kernel. This should give accurate results if --trace-origins=yes is
used, and failing that we can recompile in full debug mode to check on
insertion.

Improve drawoverhead baseline by 25% with a default build with
valgrind-dev installed (with effectively no loss of vg coverage).

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