[MergeFuncs] Don't introduce calls to weak_odr functions.

Avoid creating new calls to weak_odr functions when merging 2 functions.

Consider 2 functions below, both present in 2 modules. Without this
patch, MergeFuncs in the first module may optimize A to call B and in
the second module B to call A.

Note that the 2 optimizations are vaild in isolation, but the linker then
could pick A from module 1 (which calls B) and B from module 2 which
calls A, introducing an infinte call cycle.

There may be other linkage types we need to be more careful about as
well.

    define weak_odr hidden void @"A"(ptr %p) {
    entry:
      tail call void @"foo"(ptr %p)
      ret void
    }

    define weak_odr hidden void @"B"(ptr %p) {
    entry:
      tail call void @"foo"(ptr %p)
      ret void
    }

Author: fhahn

llvm/lib/Transforms/IPO/MergeFunctions.cpp

@@ -891,8 +891,8 @@ bool MergeFunctions::writeThunkOrAlias(Function *F, Function *G) {
 
 // Merge two equivalent functions. Upon completion, Function G is deleted.
 void MergeFunctions::mergeTwoFunctions(Function *F, Function *G) {
-  if (F->isInterposable()) {
-    assert(G->isInterposable());
+  if (F->isInterposable() || G->hasWeakODRLinkage()) {
+    assert(G->isInterposable() || G->hasWeakODRLinkage());
 
     // Both writeThunkOrAlias() calls below must succeed, either because we can
     // create aliases for G and NewF, or because a thunk for F is profitable.

llvm/test/Transforms/MergeFunc/merge-weak-odr.ll

@@ -54,7 +54,7 @@ entry:
 }
 
 declare void @zar(ptr)
-; CHECK-LABEL: define weak_odr hidden void @weak_odr_caller_of_foo_1(
+; CHECK-LABEL: define private void @0(
 ; CHECK-SAME: ptr [[P:%.*]]) {
 ; CHECK-NEXT:  [[ENTRY:.*:]]
 ; CHECK-NEXT:    tail call void @foo(ptr [[P]])
@@ -72,7 +72,7 @@ declare void @zar(ptr)
 ; CHECK-NEXT:    ret void
 ;
 ;
-; CHECK-LABEL: define hidden void @non_weak_caller_of_zar_1(
+; CHECK-LABEL: define private void @1(
 ; CHECK-SAME: ptr [[P:%.*]]) {
 ; CHECK-NEXT:  [[ENTRY:.*:]]
 ; CHECK-NEXT:    tail call void @zar(ptr [[P]])
@@ -83,7 +83,13 @@ declare void @zar(ptr)
 ;
 ; CHECK-LABEL: define weak_odr hidden void @weak_odr_caller_of_foo_2(
 ; CHECK-SAME: ptr [[TMP0:%.*]]) {
-; CHECK-NEXT:    tail call void @weak_odr_caller_of_foo_1(ptr [[TMP0]])
+; CHECK-NEXT:    tail call void @[[GLOB0:[0-9]+]](ptr [[TMP0]])
+; CHECK-NEXT:    ret void
+;
+;
+; CHECK-LABEL: define weak_odr hidden void @weak_odr_caller_of_foo_1(
+; CHECK-SAME: ptr [[TMP0:%.*]]) {
+; CHECK-NEXT:    tail call void @[[GLOB0]](ptr [[TMP0]])
 ; CHECK-NEXT:    ret void
 ;
 ;
@@ -95,6 +101,12 @@ declare void @zar(ptr)
 ;
 ; CHECK-LABEL: define weak_odr hidden void @weak_odr_caller_of_zar_2(
 ; CHECK-SAME: ptr [[TMP0:%.*]]) {
-; CHECK-NEXT:    tail call void @non_weak_caller_of_zar_1(ptr [[TMP0]])
+; CHECK-NEXT:    tail call void @[[GLOB1:[0-9]+]](ptr [[TMP0]])
+; CHECK-NEXT:    ret void
+;
+;
+; CHECK-LABEL: define hidden void @non_weak_caller_of_zar_1(
+; CHECK-SAME: ptr [[TMP0:%.*]]) {
+; CHECK-NEXT:    tail call void @[[GLOB1]](ptr [[TMP0]])
 ; CHECK-NEXT:    ret void
 ;