swift234

SILGen unconditional dynamic casts using the new
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