Use the new UNSUPPORTED() macro where possible.

Switch to using UNREACHABLE() over UNIMPLEMENTED() for switch cases that do not exist according to the spec, or for cases that should be handled by validation in an earlier stage.

Bug: b/131243109
Change-Id: Id9970b64cf248468c508e65366bc576cb03e8e58
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/29929Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Tested-by: Ben Clayton <bclayton@google.com>