[x86/SLH] Clean up helper naming for return instruction handling and
remove dead declaration of a call instruction handling helper.
This moves to the 'harden' terminology that I've been trying to settle
on for returns. It also adds a really detailed comment explaining what
all we're trying to accomplish with return instructions and why.
Hopefully this makes it much more clear what exactly is being
"hardened".
Differential Revision: https://reviews.llvm.org/D49571
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337510 91177308-0d34-0410-b5e6-96231b3b80d8