[stackprotector] Allow for copies from vreg -> vreg to be in a terminator sequence.

Sometimes a copy from a vreg -> vreg sneaks into the middle of a terminator
sequence. It is safe to slice this into the stack protector success bb.

This fixes PR16979.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191260 91177308-0d34-0410-b5e6-96231b3b80d8