swift234

Handle SIL unchecked_addr_cast conservatively when promoting to a value cast.

Unless we can prove that the input and output are layout identical we must use either trivial_bit_cast or bitwise_cast.

This changes the lowering to be more conservative for:

- Builtin.reinterpretCast
- address_to_pointer -> pointer_to_address

This also makes LoadStoreOpts more conservative in:

- forwardAddrToUncheckedCastToLd

In order to commit this without massive performance regressions, I added SILType::canBitCastAsSingleRef. When this is true for both sides
of a bitwise case, we can convert it to a RefBitCast, which has RC identity.

Swift SVN r30172