swift234

[func-sig-opts] Eliminate dead arguments that are dead besides an @owned release.

This enables dead argument elimination to be paired with @owned -> @guaranteed
optimization. It has the additional advantage of allowing us to potentially
eliminate additional retains, releases since the fact that the use is dead
implies that the lifetime of the value no longer needs to be live across the
function call.

Since dead argument elimination can be composed with @owned -> @guaranteed, I
had to modify the mangler, remangler, demangler, to be able to handle a mangling
that combines the two.

I just saw noise in the perf test suite.

rdar://21114206

Swift SVN r29966