[llvm] r215966 - IR: Reduce RAUW traffic in ConstantVector

[Duncan P. N. Exon Smith]

> On 2014-Aug-19, at 08:12, Aaron Ballman <aaron at aaronballman.com> wrote:
> 
> I have reverted from this revision to r215957 (skipping only 215961 &
> 215962 as they were unrelated) due to breaking MSVC with asserts. The
> assert is one in std::equal because it cannot be passed a nullptr,
> which your commits seemed to be doing. Since the patches all built on
> top of one another, I had to revert them in a block, sorry if this
> causes problems for you!

Sorry about that, I had no idea (and I didn't get buildbot emails).
Thanks for reverting.

I guess we should disallow using `std::equals()` if it's broken.
I'll modify `ArrayRef`, anyway, and recommit.