Use a stable-sort when combining bases

While experimenting with different algorithms for std::sort
I discovered that combine-vmovdrr.ll fails if this sort is not
stable.

I suspect that the test is too stringent in its check--the resultant
code looks functionally identical to me under both stable and unstable
sorting, but a generic fix is quite a bit more difficult to implement.

Thanks to scw@google.com for finding the proper fix.

Differential Revision: https://reviews.llvm.org/D121870