[BasicAA] Don't short-circuit non-capturing arguments
This is an alternative to D153464. BasicAA currently assumes that an unescaped alloca cannot be read through non-nocapture arguments of a call, based on the argument that if the argument were based on the alloca, it would not be unescaped. This currently fails in the case where the call is an ephemeral value and as such does not count as a capture. It also happens for calls that are readonly+nounwind+void, though that case tends to not matter in practice, because such calls will get DCEd anyway. Differential Revision: https://reviews.llvm.org/D153511
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