swift234

Implement CSE of the trio open_ext + witness_method + apply.

When we emit calls to existential methods silgen produces a sequence of the
three instructions below:

open_existential_addr %0 : $*Pingable to $*@opened("1E467EB8-...") Pingable
witness_method $@opened("1E467EB8-...") Pingable, #Pingable.ping!1
apply %3<@opened("1E467EB8-...") Pingable>(%2)

This commit adds a new CSE-like pass that finds sequences of calls to protocol
methods and reuses the first two instructions open_existential_addr and
witness_method. The optimization finds arguments that must not alias and may not
escape and combines all of the existential method calls to use the same method
lookup. The optimization handles control flow by finding the top dominating
open_existential instruction, and uses that instruction.

related to rdar://22704464.