[ValueTypes] Remove MVT::MAX_ALLOWED_VALUETYPE. NFC (#93654)

Despite the comment, this isn't used to size bit vectors or tables.
That's done by VALUETYPE_SIZE. MAX_ALLOWED_VALUETYPE is only used by
some static_asserts that compare it to VALUETYPE_SIZE.

This patch removes it and most of the static_asserts. I left one where I
compared VALUETYPE_SIZE to token which is the first type that isn't part
of the VALUETYPE range. This isn't strictly needed, we'd probably catch
duplication error from VTEmitter.cpp first.