Henry Yu 258cd1fc38 [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
2023-06-01 19:52:16 -07:00
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