[MachineOutliner] Don't outline instructions that modify SP.

This breaks the code which saves and restores LR, so we can't outline
without doing something more complicated for stack adjustment.

Found by inspection; we get lucky in most cases because getMemOpInfo
only handles STRWpost, not any other pre/post-increment forms. But it
hits a couple of artificial testcases in the tree.

Differential Revision: https://reviews.llvm.org/D46920

llvm-svn: 332529