[flang] Fix "EQ" comparison of arrays

When comparing arrays whose shapes do not conform, the contant folding
code ran into problems trying to get the value of an extent that did not
exist.  There were actually two problems.  First, the routine
"CheckConformance()" was returning "true" when the compiler was unable
to get the extent of an array.  Second, the function
"ApplyElementwise()" was calling "CheckConformance()" prior to folding
the elements of two arrays, but it was ignoring the return value.

Differential Revision: https://reviews.llvm.org/D91440