Since it is an always take, we know that the original value will always be
invalidated by the checked_cast_addr_br.
This also lets me use this to recognize simple cases of checked casts in
opt-remark-gen.
Add `llvm_unreachable` to mark covered switches which MSVC does not
analyze correctly and believes that there exists a path through the
function without a return value.
This means that:
1. SILGenPattern always borrows the object before it emits a case.
2. Any cast with this cast has a +0 result.
NOTE: That one can not use this with address types (so we assert if you
pass this checked_cast_addr_br).
NOTE: Once we have opaque values, checked_cast_br of a guaranteed value will
lower to a copy + checked_cast_addr_br (assuming the operation is a consuming
cast). To make sure this does not become a problem in terms of performance, we
will need a pass that can transform SILGenPattern +0 cases to +1 cases. This is
something that we have talked about in the past and I think it is reasonable to
implement.
This is an incremental commit towards fixing SILGenPattern for ownership.
rdar://29791263
This is necessary for correctly dealing with non-standard
ownership conventions in secondary positions, and it should
also help with non-injective type imports (like BOOL/_Bool).
But right now we aren't doing much with it.
Swift SVN r26954
