DFG optimizes out strict mode arguments tear off

[Oliver Hunt]

If i have a strict mode function:

function f(a) {
    "use strict";
    a = 5;
    return arguments[0];
}

var x = 0;
for (var i = 0; i < N; i++)
     x = x + f(1);


The result should be N, but the DFG optimizes out the eager tear off of the Arguments object, leading to arguments[0] eventually being optimized to return 5, which is incorrect.

[Filip Pizlo]

Created attachment 208962 [details]
the patch

[Filip Pizlo]

<rdar://problem/14764160>

[Filip Pizlo]

Landed in http://trac.webkit.org/changeset/154217