swift234

Add an array optimization pass that hoists make_mutable calls.

This gives us a 10x speedup on -Ofast memset, which was the original
goal. We're now within 2x of C, but the C code produces movdqu instead
of movups for twice the throughput:
<rdar://problem/17722727> Memset at -Ofast is 2x slower than C

-O3 results:
| benchmark     | baserun0 |  optrun0 |   delta | speedup |
| Memset        | 39885.00 | 33978.00 | 5907.00 |   17.4% |
| NBody         |   459.00 |   440.00 |   19.00 |    4.3% |
| QuickSort     |   456.00 |   439.00 |   17.00 |    3.9% |
| StringWalk    |   625.00 |   647.00 |   22.00 |   -3.4% |
| SmallPT       |   557.00 |   575.00 |   18.00 |   -3.1% |
| Phonebook     |  1804.00 |  1862.00 |   58.00 |   -3.1% |

Memset, NBody, and Quicksort are the ones we expected to improve.

We don't get much gain on O3 because retains/release and bounds checks
are still there.
<rdar://problem/17719220> QuickSort -O3 has retains/releases in the inner loop.

Given the Ofast results, I think the small degradations at O3 are noise.

-Ofast results:
| benchmark     | baserun0 |  optrun0 |   delta | speedup |
| Memset        |  5453.00 |   452.00 | 5001.00 | 1106.4% |
| NBody         |   772.00 |   437.00 |  335.00 |   76.7% |
| Walsh         |  1530.00 |  1096.00 |  434.00 |   39.6% |
| QuickSort     |   682.00 |   524.00 |  158.00 |   30.2% |
| Phonebook     |  1453.00 |  1561.00 |  108.00 |   -6.9% |
| Hash          |   993.00 |   958.00 |   35.00 |    3.7% |
| StringWalk    |   458.00 |   446.00 |   12.00 |    2.7% |
| StringBuilder |  1603.00 |  1568.00 |   35.00 |    2.2% |

Swift SVN r20145