Back-patch 9.4-era compiler warning fixes into older branches.

Back-patch 9.4-era compiler warning fixes into older branches.

This applies portions of commits b64b5ccb6 and b1aebbb6a to the older
branches, in hopes of getting -Werror builds to succeed there.  The
applied changes simply remove useless tests, eg checking an unsigned
variable to see if it is >= 0.  Recent versions of clang warn about
such tests by default.