Backed out changeset a5b13623631c:0200fd49efc3  (bug 838692) for frequent Linux debug test_iframe_sandbox_navigation.html timeouts

CLOSED TREE
Bug 868890 - Correctly generate barrier when accessing undefined properties of objects whose prototype also has the property, r=dvander.
Bug 869532 - Eliminate unnecessary NaN checks and emit less redundant code. r=me
Bug 869507 - Eliminate unnecessary NaN checks. r=me

Several common floating-point conditions can be implemented without using a
separate NaN test.
Bug 870095 - Prefer clobbering binary operands with no further uses. r=h4writer