beam_type: Refactor updating of the type database

The function tdb_update/2 is problematic. It does not distinguish
between assigning a register with a new value and updating information
about a register that is used a as source in a test instruction.

That was not a problem in practice when there were very few types,
but bugs started to be noticed as more types were added. (For example,
when a register was overwritten with a new value, the type for the
old value stored in the same register could linger in some cases.)

Introduce separate functions tdb_store/3 and tdb_meet/3 for assigning
a new value to a register and for updating type information for a
register referenced as as source, respectively. Also stricten
verification of the types that gets stored into the type database.

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