[SPIRV][NFC] Fix warnings for switch cases

Switch statements that cover all cases should not have a `default`
case. When a switch covers all cases and includes a `default` case,
clang emits a diagnostic. Omitting the `default` case allows the
compiler to instead emit a diagnostic on unhandled enum values.

This change removes default cases from all the places that they
shouldn't be, and adds a missing enum case for one switch statement
that wasn't covering all values.