Next round of tail call changes. Register used in a tail

Next round of tail call changes.  Register used in a tail
call must not be callee-saved; following x86, add a new
regclass to represent this.  Also fixes a couple of bugs.
Still disabled by default; Thumb doesn't work yet.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@106053 91177308-0d34-0410-b5e6-96231b3b80d8