Improve Mir2Lir::GenIntrinsic() performance.

GenIntrinsic() spends most time just comparing the method's
declaring class name to the class names we're looking for
without hiting anything. Reduce the number of comparisons
by splitting the path for java.lang.* and other calls.

Change-Id: Ic0d42ee2bde9e86b602b421c11fb2315de774a29