structures. Its default threshold is to promote things that are
smaller than 128 bytes, which is sane. However, it is not sane
to do this for things that turn into 128 *registers*. Add a cap
on the number of registers introduced, defaulting to 128/4=32.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52611 91177308-0d34-0410-b5e6-96231b3b80d8
