Fix some static analysis warnings

Fix some static analysis warnings

Most fixes are just addressing the use of (signed) enum-typed variables
as array indices.  Casting to unsigned allows us to cheaply include the
lower bound checks in the existing upper bound checks.  I would prefer
to force the underlying types of enumerations to be unsigned, but that
is a relatively new C++ feature and could have broader consequences than
I want to risk at this point.

The one other fix to asmparse.cpp just suppresses a warning that, while
technically valid, is not causing a real problem.  Perhaps this will get
better if/when #2305 is addressed.