swift234

Don't replace BB args with them self in unreachable code

CFG simplification can create unreachable code. In such code we can have loops:

 bb1(arg1): // preds: bb3
   br bb2

 bb2: // preds: bb1
   br bb3

 bb3: // preds: bb2
   br bb1(arg1)

Don't replace such block arguments (they would trigger an assert). The
unreachable code will be cleaned up later.

No test case because simplify cfg would cleanup the unreachable code before we
reach this.

rdar://22081403

Swift SVN r30854