[Coroutine] Split PHI Nodes in cleanuppad blocks in a way that obeys EH pad rules
In order to support coroutine splitting, any multi-value PHI node in a coroutine is split into multiple blocks with single-value PHI Nodes, which then allows a subsequent transform to generate reload instructions as required (i.e., to reload the value if required if the coroutine has been resumed). This causes issues with EH pads (catchswitch and catchpad) as all pads within a catchswitch must have the same unwind destination, but the coroutine splitting logic may modify them to each have a unique unwind destination if there is a PHI node in the unwind cleanuppad that is set from values in the catchswitch and cleanuppad blocks.
During splitting, if such a PHI node is detected, then create a "dispatcher" cleanuppad as well as the blocks with single-value PHI Nodes: thus the "dispatcher" is the unwind destination and it will detect which predecessor called it and then branch to the appropriate single-value PHI node block, which will then branch back to the original cleanuppad block.
Reviewed By: GorNishanov, lxfind
Differential Revision: https://reviews.llvm.org/D88059