swift234

IRGen: Peephole improvements for enum switch codegen.

If an enum has only a single tag bit, emit a cond_br instead of a switch, and if it has only one no-payload case, jump directly to the destination for that case instead of wasting time testing the bit pattern and branching to unreachable. Fixes rdar://problem/21093111.

Reapplying now that Adrian fixed the debug info issue this exposed.

Swift SVN r29202