[flang] When folding FINDLOC, convert operands to a common type

For example, FINDLOC(A,X) should convert both A and X to COMPLEX(8)
if the operands are REAL(8) and COMPLEX(4), so that comparisons
can be done without losing inforation.  The current implementation
unconditionally converts X to the type of the array A.

Differential Revision: https://reviews.llvm.org/D127030