OpDecorate should not accept any number of literal operands.

OpDecorate should not accept any number of literal operands.

This is a grammar fix.  The Decoration operand of OpDecorate (and
OpMemberDecorate) determines the remaining operands.  Don't just
allow any number of literal numbers as operands.

(The OperandVariableLiterals operand class as the last member
of the OpDecorate and OpMemberDecorate entries in in opcode.inc is
an artifact of how the spec generates the opcode descriptions. It's
not suitable for parsing those instructions.)

Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/34