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

[Duncan P. N. Exon Smith]

> On 2014-Aug-19, at 09:24, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> 
>> 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.

Worked around in ArrayRef in r215986 and recommitted in r215987.  Let me
know if anything else comes up!