[stackprotector] Added significantly longer comment to FindPotentialTailCall to make clear its relationship to llvm::isInTailCallPosition.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188770 91177308-0d34-0410-b5e6-96231b3b80d8
[stackprotector] Small Bit of computation hoisting.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188771 91177308-0d34-0410-b5e6-96231b3b80d8
[stackprotector] Small cleanup.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188772 91177308-0d34-0410-b5e6-96231b3b80d8