[flang] Support FINDLOC/MAXLOC/MINLOC with scalar mask

Previously MASK= elements were accessed in assumption that mask is an array of
input argument rank (and in combination with explicit DIM= argument we had
out-of-bounds access), but for MAXLOC/MINLOC/FINDLOC mask should be be
conformable and could be scalar.

Add new regression tests with scalar mask for verification.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D124408