Fix ExtInst parsing: no IdRef* at end

Fix ExtInst parsing: no IdRef* at end

The operands following the extended instruction literal
number are determined by the extended instruction itself.
So drop the zero-or-more IdRef pattern at the end of OpExtInst.

It's arguable whether this should actually be a grammar fix.  I've
chosen to patch this in SPIRV-Tools instead of in the grammar file.

Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/233

Also fix two test cases for OpenCL extended instructions.  These
errors of supplying too many operands are now detected.