[flang] Fix crash in location reduction folding with DIM= and scalar MASK=

The shape of an expanded scalar MASK= was incorrect in the template
code that folds FINDLOC/MAXLOC/MINLOC, leading to a vector indexing
crash when a DIM= argument is also present.

Fixes https://github.com/llvm/llvm-project/issues/64286.

Differential Revision: https://reviews.llvm.org/D157329