swift234

stdlib: Don't check for overflow in subscript accesses to ContigousArrayBuffer

We overflow checked the mulitplication in "a + sizeof(T) * i". This is not
necessary for ContingousArrayBuffer because this overflow check has happened
when we compute the size of the array during allocation of a native swift array
or a NSArray.

benchmark``````````````,``baserun0``,``optrun2``,``delta,``speedup
Memset`````````````````,``1184.00```,``487.00```,``698.00``,````````143.9%
QuickSort``````````````,``1299.00```,``1458.00``,``178.00``,````````-12.2%
SelectionSort``````````,``1027.00```,``814.00```,``213.00``,````````26.2%
StdlibSort`````````````,``1718.00```,``1587.00``,``127.00``,````````8.0%
Walsh``````````````````,``1160.00```,``1076.00``,``86.00```,````````8.1%
XorLoop````````````````,``1248.00```,``884.00```,``369.00``,````````42.0%

The  regression in quicksort is noise - i looked at the LLVM IR and the only
thing different in the graph is that we have removed the mulitplication with
overflow check (that is - we should be running faster). Same thing looking at
the assembly.

XorLoop and Memset speed up because we are now able to vectorize those loops.

Swift SVN r21218