[spirv] Remove one level of indirection: processOp to processOpImpl

We already have two levels of controls in SPIRVBase.td: hasOpcode and
autogenSerialization. The former controls whether to add an entry to
the dispatch table, while the latter controls whether to autogenerate
the op's (de)serialization method specialization. This is enough for
our cases. Remove the indirection from processOp to processOpImpl
to simplify the picture.

PiperOrigin-RevId: 259308711