Add LLVM patches to fix InstCombine loosing tbaa metadata

[yuyichao]

Seems that the patches are applied in series so there's no need to add dependencies manually anymore.

Tested on LLVM 3.7.1 and 3.8 on Linux.

Fixes #16894

Upstream review http://reviews.llvm.org/D21271, I think we can wait for this to be fixed upstream first.

[yuyichao]

The patch is merged upstream as http://reviews.llvm.org/rL272868 . I've updated the patches here to include a minor change in the final version (cast instead of static_cast).

@tkelman I guess we need a windows LLVM binary with the new patch? It would also be nice to verify that this patch fixes the problem on windows too.

[tkelman]

Do you have a test that would fail without the patch? I can prepare binaries that include this patch but am doing a bunch of other things for the next few days, when do you need them?

[yuyichao]

Do you have a test that would fail without the patch?

This fixes #16894 so the test is that both versions in that issue should vectorize.

when do you need them?

Whenever we want to have a nightly windows build that include the fix? Can this be merged before we have that?

[tkelman]

I need to explain how things work a little better apparently, the LLVM binaries that I personally build on my local machine are only used for appveyor. The buildbots that make nightlies do everything from source, so they'll pick these up as soon as they get merged. Now that we have LLVM as a shared library I could probably use the dll from the nightlies for appveyor too instead of building it by hand, but I'd still need to do something about the headers and tools like llvm-config.