this is a hack, which may or may not hang around. In short:

whimper out of doing things the Right Way, and hack up a generic
'BRCALL' instruction, that gets generated when calls are lowered.
This gets selected by hand in the DAG isel, where it gets turned
into real (i.e. in tablegen) br.call instructions.

BUG: this dies on void calls, but seems to work otherwise?
llvm-svn: 24952