[safestack] Make sure the unsafe stack pointer is popped in all cases

The unsafe stack pointer is only popped in moveStaticAllocasToUnsafeStack so it won't happen if there are no static allocas.

Fixes https://llvm.org/bugs/show_bug.cgi?id=26122

Differential Revision: http://reviews.llvm.org/D16339

llvm-svn: 259447