Move test for reduction of Math.imul to unittest.

R=bmeurer@chromium.org
TEST=compiler-unittests/JSBuiltinReducerTest.MathImul

Review URL: https://codereview.chromium.org/591373003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24144 ce2b1a6d-e550-0410-aec6-3dcde31c8c00