Assembler, bin parser: Extended instructions allowed to have too many parameters about spirv-tools

[dneto0]

Also affects GLSL.std.450:

          %1 = OpExtInstImport "GLSL.std.450"
          %blam  = OpExtInst %a %1 Round %2 %3 %5 %7 %8
          OpNop

The Round extended instruction in GLSL is only supposed to take one normal argument.

[dneto0]

I traced it to the SPIR-V core instruction grammar file: For OpExtInst, there is an argument following the LiteralExtInstInteger operand whic his IdRef with quantifier "". In the assembler and binary parser, the value of the extended instruction integer determines the expected operands, but then the IdRef-quantifier- is still on the expected-operands stack.

In other cases, e.g. OpImageFetch, the ImageOperands bit mask fully determines the operands to follow and the grammar does not include them in the ImageFetch instruction listing. It's arguable whether the same rule should apply in the case of OpExtInst. Since we've already published the grammar, and it's arguable how much checking other tools should have to do, I'm ok just patching this in the SPIRV-Tools setup phase.