The logic for different special cases of type casting is spread over multiple places currently. This patch simply re-factors some of that code (folding of of type casts using statically known protocol conformances) and moves it into one central place, which makes it easier to maintain. Plus, it allows other clients of DynamicCasts benefit from it as well, e.g. the inliner can use this now. NFC.
Swift SVN r25486
unconditional_dynamic_cast_addr instruction.
Also, fix some major semantic problems with the
existing specialization of unconditional dynamic
casts by handling optional types and being much
more conservative about deciding that a cast is
infeasible.
This commit regresses specialization slightly by
failing to turn indirect dynamic casts into scalar
ones when possible; we can fix that easily enough
in a follow-up.
Swift SVN r19044
