We need to know how to generate correct SPIRV for cases like
OpConstant %int64 42 since the current parser will encode the 42 as a
32-bit value incorrectly.
This change is the first of a pair. This one tracks types, and makes
sure that OpConstant and OpSpecConstant are only ever called with
Integer or Float types, and OpSwitch is only called with integer
generating values.
Move the definition of spv_instruction_t to an internal
header file, since it now depends on C++ and is not
used by the external interface.
Use a std::vector<uint32_t> in spv_instruction_t
instead of a fixed size array.
