Turn DCHECKs into return false in ReplaceUninitializedStringPhis.

Until https://android-review.googlesource.com/c/platform/art/+/754691,
gets resolves, temporarily change
the method to return whether it fits the expecations, and bail
to interpreter if not.

Test: test.py
bug: 112537407
Change-Id: I60f0c3bc75f83b3b8a6c19e733c64440881a5a76