Give the always-inliner its own custom filter. It shouldn't have to pay

Give the always-inliner its own custom filter. It shouldn't have to pay
the very high overhead of the complex inline cost analysis when all it
wants to do is detect three patterns which must not be inlined. Comment
the code, clean it up, and leave some hints about possible performance
improvements if this ever shows up on a profile.

Moving this off of the (now more expensive) inline cost analysis is
particularly important because we have to run this inliner even at -O0.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153814 91177308-0d34-0410-b5e6-96231b3b80d8