Add test for fn pointer duplication.

Author: cjgillot

tests/ui/mir/auxiliary/static_fnptr.rs

@@ -0,0 +1,10 @@
+//@ compile-flags:-O
+
+fn foo() {}
+
+pub static ADDR: fn() = foo;
+
+#[inline(always)]
+pub fn bar(x: fn()) -> bool {
+    x == ADDR
+}

tests/ui/mir/static_fnptr.rs

@@ -0,0 +1,14 @@
+//@ run-pass
+//@ compile-flags:-Cno-prepopulate-passes -Copt-level=0
+//@ aux-build:static_fnptr.rs
+
+extern crate static_fnptr;
+use static_fnptr::{ADDR, bar};
+
+fn baz() -> bool {
+    bar(ADDR)
+}
+
+fn main() {
+    assert!(baz())
+}