Remove useless int64 range checks on BIGINT sequence MINVALUE/MAXVALUE values

Hi,

Sequence MINVALUE/MAXVALUE values are read into "int64" variables and
then range-checked according to the sequence data-type.
However, for a BIGINT sequence, checking whether these are <
PG_INT64_MIN or > PG_INT64_MAX always evaluates to false, as an int64
can't hold such values.
I've attached a patch to remove those useless checks.
The MINVALUE/MAXVALUE values are anyway int64 range-checked prior to
this, as part of conversion to int64.

Regards,
Greg Nancarrow
Fujitsu Australia