According to the SPIRV Spec (2.16.1):
* There is at least one OpEntryPoint instruction, unless the Linkage
capability is being used.
* No function can be targeted by both an OpEntryPoint instruction and an
OpFunctionCall instruction.
Also updated unit tests to includ OpEntryPoint.
These rules are under "Data Rules" in 2.16.1 (Universal Validation
Rules) part of the SPIR-V 1.1 Specification document:
* Scalar floating-point types can be parameterized only as 32 bit, plus
any additional sizes enabled by capabilities.
* Scalar integer types can be parameterized only as 32 bit, plus any
additional sizes enabled by capabilities.
* Vector types can only be parameterized with numerical types or the
OpTypeBool type.
* Matrix types can only be parameterized with floating-point types.
* Matrix types can only be parameterized as having only 2, 3, or 4
columns.
* Specialization constants (see Specialization) are limited to integers,
Booleans, floating-point numbers, and vectors of these.
