[flang] Fix common block size extension mistake in D109156

https://reviews.llvm.org/D109156 did not properly update the case where
the equivalence symbol appearing in the common statement is the
"base symbol of an equivalence group" (this was the only case that previously
worked ok, and the patch broke it).

Fix this and add a test that actually uses this code path.

Differential Revision: https://reviews.llvm.org/D109439