Instruction lookup succeeds if it's enabled by a capability

Also add a corresponding check for capabilities in the validator.

Update previously existing test cases where an instruction used to fail
assembling because of a version check, but now they succeed because the
instruction is also guarded by a capability.  Now it should assemble.
Add tests to ensure that capabilities are checked appropriately.

The explicitly reserved instructions OpImageSparseSampleProj*
now assemble, but they fail validation.

Fixes #1624