Improve String.indexOf() intrinsics.

If the code point input is a char, we don't need the slow
path. Also improve the slow-path check (if we do need it)
on arm and arm64 to avoid loading 0xffff into a register.

Bug: 28330359
Change-Id: Ie6514c16126717bb0b11e3c7ab2b60eaa70fed4c
6 files changed