Remove conflicting attributes before adding deduced readonly/readnone

[dotdash]

Summary

In case of functions that have a pointer argument and only pass it to
each other, the function attributes pass deduces that the pointer should
get the readnone attribute, but fails to remove a readonly attribute
that may already have been present.

lib/Transforms/IPO/FunctionAttrs.cpp

for (unsigned i = 0, e = ArgumentSCC.size(); i != e; ++i) {
         ReadAttr = Attribute::ReadOnly;
         continue;
       }
       ReadAttr = K;
       break;
     }
 
     if (ReadAttr != Attribute::None) {
-      AttrBuilder B;
+      AttrBuilder B, R;
       B.addAttribute(ReadAttr);
+      R.addAttribute(Attribute::ReadOnly)
+        .addAttribute(Attribute::ReadNone);
       for (unsigned i = 0, e = ArgumentSCC.size(); i != e; ++i) {
         Argument *A = ArgumentSCC[i]->Definition;
+        // Clear out any existing attributes.
+        A->removeAttr(AttributeSet::get(A->getContext(), A->getArgNo() + 1, R));
         A->addAttr(AttributeSet::get(A->getContext(), A->getArgNo() + 1, B));
         ReadAttr == Attribute::ReadOnly ? ++NumReadOnlyArg : ++NumReadNoneArg;
         Changed = true;
       }
     }
   }
 
   return Changed;

test/Transforms/FunctionAttrs/readnone.ll

+; RUN: opt < %s -functionattrs -S | FileCheck %s
+
+; CHECK: define void @bar(i8* nocapture readnone)
+define void @bar(i8* readonly) {
+  call void @foo(i8* %0)
+    ret void
+}
+
+; CHECK: define void @foo(i8* nocapture readnone)
+define void @foo(i8* readonly) {
+  call void @bar(i8* %0)
+  ret void
+}