swift234

Implement a micro-optimization to the let-else SILGen for enum matches:
don't run the cleanup for the enum if the failure path can only contain
trivial enum element matches.  For example, on:

class Base {}

func test_is(a : Base?) {
  let x? = a else { abort() }
}

we previously emitted:

  switch_enum %0 : $Optional<Base>, case #Optional.Some!enumelt.1: bb2, default bb1 // id: %3

bb1:                                              // Preds: bb0
  release_value %0 : $Optional<Base>              // id: %4
  br bb3                                          // id: %5


which is silly because the release_value is always dynamically a no-op (the 
.None element of an enum is always trivial).



Swift SVN r27460