[IR] First pass cleanup of ReceiveArg* instructions

* Removed specialized 1.8 and 1.9 ReceiveOptArg and ReceiveRestArg
  instructions -- it is possible to use the same version for both
  modes. In addition, the same versions are usable in 2.0 mode which
  eliminates more specialized versions.

* TODO: Fix up keyword arg instructions and interpreter to pass
  used keyword arg information into the ReceiveArg* interpretation.