Fix case where block has no predecessor for StringChange.

Removes part that checks for throwing half of instruction. It's no
longer necessary. Also adds regression test.

Bug: 21902684

(cherry-picked from commit 3e91a44bc9063f7f69b5415e3cf162991f73283f)

Change-Id: I8d7308ff227816fb475341c2b74ed84d5695a2f5
4 files changed