Revert "Revert "Move rewritten StringFactory call results into dex registers for deopt""

Potential gc points can make the result value stale. We now set the result value
to null proactively once it's moved to shadow frame registers. IsStringInit()
is written in a way that does string comparison instead of requiring method
resolution so that it doesn't have a gc point. Also we don't cache the callee
method during frame unwinding since the method may be rewritten already.

Bug: 28555675

Change-Id: Ic51511a4a0fc84a852d8d907f91e7835f49ac478