swift234

SILGen: Factor abstraction change from branch in checked cast emission.

<rdar://problem/14835992> happens because, when we emitted a chain of checked casts for 'is' patterns in a 'switch', we would emit abstraction changes for each individual cast that required them. This is both redundant and makes cleaning up messier, because the number of branches with different stack heights increases. Instead, let's emit one abstraction change up front for all of the casts. No functionality change yet, but this is a step on the way to fixing <rdar://problem/14835992>.

Swift SVN r9921