[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