[FuzzMutate] Handle BB without predecessor, avoid insertion after `musttail...

[FuzzMutate] Handle BB without predecessor, avoid insertion after `musttail call`, avoid sinking token type

FuzzMutate didn't consider some corner cases and leads to mutation failure when mutating some modules.
This patch fixes 3 bugs:

- Add null check when encountering basic blocks without predecessor to avoid segmentation fault
- Avoid insertion after `musttail call` instruction
- Avoid sinking token type

Unit tests are also added.

Reviewed By: Peter

Differential Revision: https://reviews.llvm.org/D151936