[CodeGen] Remove dead FastISel code after SDAG emitted a tailcall.

When SDAGISel (top-down) selects a tail-call, it skips the remainder
of the block.

If, before that, FastISel (bottom-up) selected some of the (no-op) next
few instructions, we can end up with dead instructions following the
terminator (selected by SDAGISel).

We need to erase them, as we know they aren't necessary (in addition to
being incorrect).

We already do this when FastISel falls back on the tail-call itself.
Also remove the FastISel-emitted code if we fallback on the
instructions between the tail-call and the return.

llvm-svn: 296552