[BasicAA] Don't short-circuit non-capturing arguments

[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