Fix wrong assumption in RemoveInstructionsAsUsersFromDeadBlocks.

It can be called in a situation where dead blocks can have phis:
the DCE pass would re-build the dominator tree and RemoveInstructionsAsUsersFromDeadBlocks
used to assume there cannot be phis.

Test: 695-simplify-throws
Bug: 131174581
Change-Id: I853956482487a8c7c8fb99499aef318d099754e4
4 files changed