Fix StringBuilder append assumptions.

Do not rely on use ordering, it can be different because of
SimplifyReturnThis(); just use HBackwardInstructionIterator.
Instead of checking that we find a StringBuilder.toString(),
check that it is the invoke we're trying to simplify.

Add regression test 699-checker-string-append2.

Test: testrunner.py --host --jvm -t 699-checker-string-append2
Bug: 19575890
Bug: 146014745
Change-Id: I7b16f376c16ba5a4107e9718e0acf17d82280f54
6 files changed