Fix PR#22427. The implementation of inplace_merge had a \'small data set\'...

Fix PR#22427. The implementation of inplace_merge had a \'small data set\' optimization; if either half of the merge was small (i.e, less than 9 items), it did an inplace merge rather than allocating a buffer and doing a faster/smarter merge. However, this failed to satisfy the complexity requirements in the standard. Remove that code. Add tests to check the complexity, and add the same tests for std::merge, since we are in that section of the test suite anyway.

llvm-svn: 227811