Fix PR2369 by making scalarrepl more careful about promoting

Fix PR2369 by making scalarrepl more careful about promoting
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