Fix read of uninitialized memory in InlineIndexOf

The are two flavors of IndexOf that we treat as an intrinsic: a
zero-based verion with 2 args and a 3-arg version that also takes
a start position.  The same code is used for both, but Valgrind
reminded us that we shouldn't try loading a RegLocation for the
non-extent 3rd arg in the 2 argument version.

We got lucky in that the bug was benign - the generated code would
still be correct.

Change-Id: I0bc7798c8034d35007ffe6d6d62f9ceb91fc44fd