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

(cherry picked from commit 2cc9d343550d22e08536e0077893e6d86647c91a)

Change-Id: I674384382b2c35175ca82c9e7d0d9ccbe6c1ab02
Merged-In: I853956482487a8c7c8fb99499aef318d099754e4