[flang] Fix MAXLOC/MINLOC when MASK is scalar .FALSE.

When passing a scalar .FALSE. as the MASK argument to MAXLOC, we were getting
bad memory references.  We were falling into the code intended when the MASK
argument was missing.

I fixed this by checking for a scalar MASK with a .FALSE. value and
setting the result to all zeroes in that case.  I also added tests for
MAXLOC and MINLOC with scalar values of .TRUE. and .FALSE. for the MASK
argument.

I also special cased situations where the MASK argument is a scalar with
a .TRUE. value and passed along a nullptr in such cases.

Along the way, I eliminated the unused "chars" argument from the constructor
for ExtremumLocAccumulator.

Differential Revision: https://reviews.llvm.org/D124484