Remove 'if()' branches from i64 add and subtract code.

Rewrites i64 add and sub code to not contain if()s, instead relies on bool -> int cast to produce 1 or 0, and add or subtract that to carry and borrow.

Passes asm1 test suite.

Profiled to be minorly faster than the branching version.

After: https://dl.dropboxusercontent.com/u/40949268/emcc/i64sub_1.html
Before: https://dl.dropboxusercontent.com/u/40949268/emcc/i64sub_2.html