[ARM] Glue register copies to tail calls.

This generally follows what other targets do. I don't completely
understand why the special case for tail calls existed in the first
place; even when the code was committed in r105413, call lowering
didn't work in the way described in the comments.

Stack protector lowering breaks if the register copies are not glued
to a tail call: we have to insert the stack protector check before
the tail call, and we choose the location based on the assumption
that all physical register dependencies of a tail call are adjacent
to the tail call. (See FindSplitPointForStackProtector.) This is sort
of fragile, but I don't see any reason to break that assumption.

I'm guessing nobody has seen this before just because it's hard to
convince the scheduler to actually schedule the code in a way that
breaks; even without the glue, the only computation that could
actually be scheduled after the register copies is the computation of
the call address, and the scheduler usually prefers to schedule that
before the copies anyway.

Fixes https://bugs.llvm.org/show_bug.cgi?id=41417

Differential Revision: https://reviews.llvm.org/D60427