spirv: Implement OpPtrEqual, OpPtrNotEqual and OpPtrDiff
These are new operations from SPIR-V 1.4. Only the two last patches are new in this MR, the others come from:
- New headers for SPIR-V 1.4 (merged)
- !823 (merged) make it OK for derefs to be used as phi sources, a case which surfaces in a few tests using these new operators
- !781 (merged) provides support for real values in NULL pointers, which can be used together with OpPtrEqual and OpPtrNotEqual