Operand lookup succeeds if it's enabled by a capability

- Fix tests for basic group operations (e.g. Reduce) to allow for
new capabilities in SPIR-V 1.3 that enable them.
- Refactor operand capability check to avoid code duplication and
to put all checks that don't need table lookup before any table
lookup.
- Test round trip assembly/disassembly support for extension
SPV_NV_viewport_array2
- Test assembly and validation of decoration ViewportRelativeNV

Fixes #1596