C++: Normalize TInstruction in raw IR

This definition was denormalized to the extent that an instruction was
defined in terms of the six main attributes it could be queried for.
This made it possible to do multi-column joins on those six attributes,
but it doesn't appear that this feature was useful in practice. The main
multi-column join that was in use was on the pair of
(`TranslatedElement, InstructionTag`), but the `TranslatedElement` was
not part of the `TInstruction`.

This commit changes `TInstruction` to be defined in terms of what it's
_built from_ (`TranslatedElement, InstructionTag`) instead. This makes
it possible to do multi-column joins on those two components, and then
there are separate predicates (usually with two columns) to query
instruction attributes, replacing the many uncached projections from
`MkInstruction` that were generated before.

An immediate advantage is that an `Expr` with multiple types will no
longer give rise to multiple `Instruction`s, fixing most of the errors
from the sanity query `ambiguousSuccessors`. The code inside
`IRConstruction.qll` becomes simpler and hopefully faster as there is no
longer a translation from `TranslatedElement` to `Locatable` and back
again.

Author: jbj