swift234

Allow hoisting of uniqueness check in array.append

At -O I see about 10% improvement on Havlak, and 40% improvement on the recently
added array append benchmarks.

COWTree regresses by 15% because we hoist an append that is conditionally
executed. If we were to disallow hoisting of conditionally executed benchmarks
we would see big loses accross our benchmarks (for example, we would not hoist
out of nested loops anymore). So we will have to live with this regression.

rdar://17140639

Swift SVN r31120