Hydrogen binop improvements

- Truncate oddball if possible.
- Support for StringAdd with only one String argument.
- Use constructor macro for HMul.
- Add ForceNumberType for HydrogenStubs to enforce input representations.

BUG=
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/23503058

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16849 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Turn off GcStress for another mjsunit test that asserts heavily on array
ElementsKind.

BUG=
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/23983041

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16850 ce2b1a6d-e550-0410-aec6-3dcde31c8c00