... along with a message saying what switch case wasn't handled.

Helps identify what needs fixing for the daily Regres reports.

Change-Id: I79448135fc84190d6e803e90e7995ae83058f663
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28400Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>