Fix overly aggressive type merging.

If we find out that two types are *not* isomorphic, we learn nothing about
opaque sub types in both the source and destination.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222727 91177308-0d34-0410-b5e6-96231b3b80d8

test/Linker/type-unique-opaque.ll

@@ -0,0 +1,16 @@
+; RUN: llvm-link -S %s %p/Inputs/type-unique-opaque.ll | FileCheck %s
+
+; Test that a failed attempt at merging %u2 and %t2 (for the other file) will
+; not cause %u and %t to get merged.
+
+; CHECK: %u = type opaque
+; CHECK: define %u* @g() {
+
+%u = type opaque
+%u2 = type { %u*, i8 }
+
+declare %u2* @f()
+
+define %u* @g() {
+  ret %u* null
+}