Linker: Simplify test/Linker/dicompositetype-unique.ll, NFC

Simplify the test logic a little, sharing logic between the two linking
directions by specifying -check-prefix multiple times.  Now it's more
obvious what's hte same and different between the two directions, and
there is less CHECK duplication.  This is a prep for expanding the test.

llvm-svn: 266773