Add an operand to memory intrinsics to denote the "tail" marker.

We need to propagate this information from the IR in order to be able to safely
do tail call optimizations on the intrinsics during legalization. Assuming
it's safe to do tail call opt without checking for the marker isn't safe because
the mem libcall may use allocas from the caller.

This adds an extra immediate operand to the end of the intrinsics and fixes the
legalizer to handle it.

Differential Revision: https://reviews.llvm.org/D68151

llvm-svn: 373140