[flang] Remove redundant reallocation

The MaxMinHelper used to implement MIN and MAX for character types would
reallocate the accumulator whenever the number of characters in it was
different from that in the other input. This is unnecessary if the
accumulator is already larger than the other input. This patch fixes the
issue and adds a unit test to make sure we don't reallocate if we don't
need to.

Differential Revision: https://reviews.llvm.org/D101984