Slience compiler warnings for NV, [IU]V compare

Slience compiler warnings for NV, [IU]V compare

These were occurring on FreeBSD smokes.

warning: implicit conversion from 'IV' (aka 'long') to 'double' changes value from 9223372036854775807 to 9223372036854775808 [-Wimplicit-int-float-conversion]

9223372036854775807 is IV_MAX.  What needed to be done here was to use
the NV containing IV_MAX+1, a value that already exists in perl.h

In other instances, simply casting to an NV before doing the comparison
with the NV was what was needed.

This fixes #18328