swift

02939d16 - Fix SILCombine metatype cast optimization to avoid extending lifetimes.

Fix SILCombine metatype cast optimization to avoid extending lifetimes. This cannot be correctly done as a SILCombine because it must create new instructions at a previous location. Move the optimization into CastOptimizer. Insert the new metatype instructions in the correct spot. And manually do the replaceAllUsesWith and eraseInstruction steps. Fixes <rdar://problem/46746188> crash in swift_getObjCClassFromObject. (cherry picked from commit 65c95dd8409cfdf0623cf27cc47bc44da733f4e0)