Make CopyForwarding more conservative with destination addresses.
Make CopyForwarding more conservative with destination addresses.

AFAICT, this does not fix any existing bug, but eliminates unverified
assumptions about well-formed SIL, which could be broken by future
optimization.

Forward: The optimization will replace all in-scope uses of the
destination address with the source. With this change we will be sure
not eliminate writes into a destination address unless the destination
is an AllocStackInst. This hasn't been a problem in practice because
the optimization requires an in-scope deinit of the destination
address, which can't happen on typical address projections.

Backward: The optimization will replace in-scope uses of the source
with the destination. With this change we will be sure not to write
into the destination location prior to the copy unless the destination
is an AllocStackInst. This hasn't been a problem in practice because
the optimization requires the copy to be an initialization of the
address, which can't happen on typical address projections.

This change prevents both optimizations without an obvious guarantee
that any dependency on the destination address will manifest as a
SIL-level dependence on the address producer. For example,
init_enum_data_addr would not qualify because it simply projects an
address within a value that may have other dependencies.

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