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.
For fulfilling this purpose, the |opcode| field in the
|spv_parsed_instruction_t| struct is changed to of type uint16_t.
Also add functions to query the information of a given SPIR-V
target environment.
