Enhance TableGen to emit code for FastISel of opcodes with variadic return types...

Enhance TableGen to emit code for FastISel of opcodes with variadic return types without slowing down opcodes that are not variadic.  No such opcodes are currently generated, but in theory it should be a matter of just hitting the switch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@55347 91177308-0d34-0410-b5e6-96231b3b80d8