Convert sampled consumers to being Instructions instead of IDs (#2464)

Changing the stored value for a sampled image consumer to be the
instruction instead of result ID, since not all instructions have
result IDs. Using result IDs led to a potential crash when using
OpReturnValue, which doesn't have a result ID. OpReturnValue is not a
legal consumer, but the validator needs to look at the instruction to
determine this, thus storing the pointer to the instruction, instead
of trying to fetch the pointer using the instruction.

Issue #1528 covers fixing the check.

Fixes #2463