[flang] Fix spurious error with COMMON and EQUIVALENCE

f18 emits an error message when two objects related by EQUIVALENCE
to a third are specified as members of a COMMON block.  This is not
always a sign of an error, however; it is possible for multiple objects
in a COMMON block to all be equivalenced to distinct offsets in another
object in a way that is consistent.  So refine the check.

Differential Revision: https://reviews.llvm.org/D134485