have the mc lowering process handle a few tail call forms, lowering them to

have the mc lowering process handle a few tail call forms, lowering them to
jumps where possible and turning the TAILCALL marker in the instruction
asm string into a proper comment.

This eliminates a FIXME and is on the path to finishing:
rdar://7639610 - eliminate encoding and asm info for TAILJMPd TAILJMPr TAILJMPn, etc.

However, I can't eliminate the encodings for these instructions because the JIT
still exists and has its own copy of the encoder, sigh.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@107946 91177308-0d34-0410-b5e6-96231b3b80d8