Don't merge tiny functions.

It's silly to merge functions like these:

define void @foo(i32 %x) {
  ret void
}

define void @bar(i32 %x) {
  ret void
}

to get

define void @bar(i32) {
  tail call void @foo(i32 %0)
  ret void
}

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191786 91177308-0d34-0410-b5e6-96231b3b80d8