[flang] Handle reversed bounds and negative length in inlined allocation

ALLOCATE statement allows reversed bounds (see Fortran 2018 9.7.1.2
point 1) in which case the extents are zero.

The same applies for the character length provided in the type spec that
can be negative. In which case the new length is zero.

Use genMaxWithZero to deal with these cases.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: jeanPerier, PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D127617



Co-authored-by: Jean Perier <jperier@nvidia.com>