swift234

[g-arc-opts] Teach the ARC optimizer how to strip off ref count equivalent operations such as casts and ref count identity preserving typed geps.

Now that the ARC optimizer does not rely on stripCasts I also added
unchecked_trivial_bit_cast to stripCasts.

This and r21164 give the following speedups > 10%:

Ary      55.95%
MatMul   36.71%
Ary2     34.97%
Richard  32.08%
PrimeNum 15.87%

<rdar://problem/17456455>

Swift SVN r21240