Reimplement BuildResultOperands to be in terms of the result instruction's

operand list instead of the operand list redundantly declared on the alias
or instruction.

With this change, we finally remove the ins/outs list on the alias.  Before:
  def : InstAlias<(outs GR16:$dst), (ins GR8 :$src),
                  "movsx $src, $dst",
                  (MOVSX16rr8W GR16:$dst, GR8:$src)>;
After:
  def : InstAlias<"movsx $src, $dst",
                  (MOVSX16rr8W GR16:$dst, GR8:$src)>;

This also makes the alias mechanism more general and powerful, which will
be exploited in subsequent patches.

llvm-svn: 118329