pgsql: Fix incidental warnings from cpluspluscheck.

Fix incidental warnings from cpluspluscheck.

Remove use of "register" keyword in hashfn.c.  It's obsolescent
according to recent C++ compilers, and no modern C compiler pays
much attention to it either.

Also fix one cosmetic warning about signed vs unsigned comparison.

Discussion: https://postgr.es/m/20518.1559494394@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/232720be9b6412ec2b6bee405299bcbbbe700f0b

Modified Files
--------------
src/backend/utils/hash/hashfn.c   | 16 ++++++++--------
src/include/utils/expandeddatum.h |  2 +-
src/include/utils/hashutils.h     |  6 +++---
3 files changed, 12 insertions(+), 12 deletions(-)