A new TableGen feature! (Not turned on just yet.)

   InstAlias<{alias}, {aliasee}>;

The InstAlias instruction should be able to go from the MCInst to the
{alias}. All of the information is there to match the MCInst with the
{aliasee}. From there, it's a simple matter to emit the {alias}, with the
correct operands from the {aliasee}.

The code this patch generates can be used by the InstPrinter to automatically
print out the alias without having to write special C++ code to handle the
situation.

This is a WIP, and therefore are several limitations. For instance, it cannot
handle AsmOperands at the moment. It also doesn't know what to do when two
{alias}es match the same {aliasee}. (Currently, it just ignores those two cases
and allows the printInstruction method to handle them.)

llvm-svn: 126538