[TableGen][GlobalISel] Use `GIM_SwitchOpcode` in Combiners (#66864)

The call to `initOpcodeValuesMap` was missing, causing the MatchTable to
(unintentionally) not emit a `SwitchMatcher`. Also adds other code
imported from `GlobalISelEmitter.cpp` to ensure rules are sorted by
precedence as well.

Overall this improves GlobalISel compile-time performance by a
noticeable amount. See #66751