Revert "Fix testcase for attribute ordering."

This reverts commit 997c6516ca161073a1d516ebca7c0ca7722f64e2.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175248 91177308-0d34-0410-b5e6-96231b3b80d8
Revert "Fix testcase for attribute ordering."

This reverts commit 58f20a3cbfca7384fe5e25e095f18572736a4792.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175249 91177308-0d34-0410-b5e6-96231b3b80d8
Revert "Simplify the attributes '<' comparison function."

This reverts commit 82c101153fe7b35bce48781fab038e1b8f31a7bd.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175250 91177308-0d34-0410-b5e6-96231b3b80d8
BBVectorize: Cap the number of candidate pairs in each instruction group

For some basic blocks, it is possible to generate many candidate pairs for
relatively few pairable instructions. When many (tens of thousands) of these pairs
are generated for a single instruction group, the time taken to generate and
rank the different vectorization plans can become quite large. As a result, we now
cap the number of candidate pairs within each instruction group. This is done by
closing out the group once the threshold is reached (set now at 3000 pairs).

Although this will limit the overall compile-time impact, this may not be the best
way to achieve this result. It might be better, for example, to prune excessive
candidate pairs after the fact the prevent the generation of short, but highly-connected
groups. We can experiment with this in the future.

This change reduces the overall compile-time slowdown of the csa.ll test case in
PR15222 to ~5x. If 5x is still considered too large, a lower limit can be
used as the default.

This represents a functionality change, but only for very large inputs
(thus, there is no regression test).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175251 91177308-0d34-0410-b5e6-96231b3b80d8