Fix for the bug with gcc4.7: incorrect usage of ArrayRef

This fixes big number of comp fails for generic-4 target when ISPC is built by gcc 4.7 with optimizations. gcc 4.2 and other compilers are not affected.

Before the fix I had 166 comp fails / 37 run fails with generic-4 (gcc4.7) and only 2 comp fails / 38 run fails when ISPC built with clang.
After the fix, it’s the same for clang and gcc4.7: 2 comp fails / 38 run fails.

The fix itself looks pretty stupid from the first glance, but this actually fixes incorrect usage of LLVM interface. ArrayRef expects as a parameter “(const T &OneElt)”. I.e. it expects a reference to an object, it doesn’t own the object, but expects that it lives long enough and it keeps a reference. The old code provided *temporary value*, which live range was not long enough. So compiler may optimize it away. gcc4.7 at O2 does optimize this temporary object, while other compilers don’t (at the moment).