[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

llvm-svn: 337510