Adding validation code for OpTypeStruct.

Adding validation code for OpTypeStruct.

According to the Data Rules section of 2.16.1. Universal Validation
Rules of the SPIR-V Spec:

Forward reference operands in an OpTypeStruct
* must be later declared with OpTypePointer
* the type pointed to must be an OpTypeStruct
* had an earlier OpTypeForwardPointer forward reference to the same <id>