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

[Duncan P. N. Exon Smith]

> On 2014-Aug-19, at 08:51, David Majnemer <david.majnemer at gmail.com> wrote:
> 
>> On Tuesday, August 19, 2014, 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. 
> 
> MSVC's STL is over zealous in this regard, it's a known bug; they really do not like nullptr iterators in some places. :/

I documented this in r215988.