intel/ir: Represent physical and logical subsets of the CFG.

This represents two control flow graphs in the same cfg_t data
structure: The physical CFG that will include all possible control
flow paths the EU can physically take, and the logical CFG restricted
to the control flow paths that exist in the original scalar program.
The latter is a subset of the former because in case of divergence the
SIMD vectorized program will take control flow paths that aren't part
of the original scalar program.

The bblock_link constructor and bblock_t::add_successor() now take a
"kind" parameter that specifies whether the edge is purely physical or
whether it's part of both the logical and physical CFGs (a logical
edge is of course always guaranteed to be in the physical CFG as
well).  bblock_t::is_predecessor_of() and ::is_successor_of() also
take a kind parameter specifying which CFG is being queried.  The '~>'
notation will be used now in order to represent purely physical edges
in IR dumps.

This commit doesn't actually add nor remove any edges from the CFG
(the only edges marked as purely physical here are the two WHILE loop
ones that already existed).  Optimization passes should continue using
the same (incomplete) physical CFG they were using before until
they're fixed to do something smarter in a later commit, so this
shouldn't lead to any functional changes.

v2: Remove tabs from lines changed in this file (Caio).

Reviewed-by: Jordan Justen <[email protected]>
Reviewed-by: Caio Marcelo de Oliveira Filho <[email protected]>

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