These used to assert.
Instead of asserting, let the compiler warn about unhandled cases - there's already a `return "<unknown>"` statement below the switch.

Credit to Dan Sinclair who did all the hard work here.

Bug: None - maintenance fix for legacy code
Change-Id: I010a1d6228961a80a15523ff1a2c6db522b7756d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40833
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>