[llvm-commits] [patch][pr11202] Handle BB with address taken being deleted from codegen

[Rafael Ávila de Espíndola]

We currently handle the following cases when removing a BB that has its 
address taken

* An IL pass is deleting it, and no references to it were emitted so 
far. In this case replaceAllUsesWith takes care of everything.
* An IL pass is deleting it, but a reference to it was already emitted. 
When emitting the reference we created a CallbackVH, so now we get 
notified about the deleting and remember to produce a dummy label for it .

The case we are not handling in this bug is a BB that is referenced from 
another function or global variable being remove by a codegen pass. 
There are two problems

* replaceAllUsesWith will not work once we are in Codegen, so we will 
need to use dummy labels for both cases.
* MachineBasicBlock are not Values, so we cannot use CallbackVH.

What this patch does is add a method to MMI that Codegen can use to 
record that a MBB is being deleted. This is handled almost the same way 
we handle and IL BB being deleted, but we have to keep the hash table 
entry since the IL level BB will not be delete and can still be in use.

Cheers,
Rafael
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