Restructure `InstructionSimplifierVisitor::VisitAdd()`.

Address a comment from
    https://android-review.googlesource.com/2565351 .

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: Ie91fbfe54f9f067b79d3c8fb5587c44b4d531ef9
1 file changed