Unused function declarations within functions not removed

[kzc]

The unused function foo is removed at top level scope, but not at function scope.

$ node bin/rollup -v
rollup version 0.36.3

$ cat wrap.js

function foo() { console.log("FOO"); /* is removed */ }
function baz() { console.log("BAZ"); }
function boo() { /* is removed */ }
baz();

function quux() {
    function foo() { console.log("FOO"); /* should be removed */ }
    function baz() { console.log("BAZ"); }
    function boo() { /* should be removed */ }
    baz();
}
quux();

$ node bin/rollup wrap.js

function baz() { console.log("BAZ"); }
baz();

function quux() {
    function foo() { console.log("FOO"); /* should be removed */ }
    function baz() { console.log("BAZ"); }
    function boo() { /* should be removed */ }
    baz();
}
quux();

Note: this is not a dupe of #977.