Replace the runtime `warning` with something that will actually crash
us, and add stub implementations of OpLabel and OpReturn to allow
trivial shaders to work.

There's little point in continuing beyond an unimplemented instruction
-- if you get lucky, you hit a crash on use of the instruction's result.
If you get unlucky, you wander off into undefined behavior.

Change-Id: I3b222ea74446754276096c81fc3669c311872316
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/26088Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>