Ignore "push/pop {sp}" in emulation based unwinding

These instructions confusing the unwind code because in case of a
push it assumes that the original valu of a register is pushed to
the stack what is not neccessarily true in case of SP. The same is
true for the pop (in the opposite way).

Differential revision: http://reviews.llvm.org/D10806

llvm-svn: 241051