swift234

[DCE] Add unreachable code warning to DCE pass.

This commit teaches DCE to warn about unreachable code. The general approach
is to record the BasicBlocks that become unreachable as we are folding
conditional branches. Later, when all the reachability info is available,
we walk though these blocks and issue warnings on the ones that contain SIL
instructions that correspond to user code. We determine if a SIL instruction
corresponds to user code by looking at the SILLocations.

TODO:
 - Note, that folding conditions is not as strong right now. Specifically,
   folding 'x == 0' for Ints is blocked by generics specialization. This limits
   the applicability of the warning on real code.
 - We need to extend this to other foldable terminators (not only conditional branch).
 - We could print a note telling the user which condition has been folded in
   order to get the unreachable code.

Swift SVN r9506