i965: Preserve CFG when deleting dead control flow.

i965: Preserve CFG when deleting dead control flow.

This pass deletes an IF/ELSE/ENDIF or IF/ENDIF sequence, or the ELSE in
an ELSE/ENDIF sequence.

In the typical case (where IF and ENDIF) aren't the only instructions in
their basic blocks, we can simply remove the instructions (implicitly
deleting the block containing only the ELSE), and attempt to merge
blocks B0 and B2 together.

B0: ...
    (+f0) if(8)
B1: else(8)
B2: endif(8)
    ...

If the IF or ENDIF instructions are the only instructions in their
respective basic blocks (which are deleted by the removal of the
instructions), we'll want to instead merge the next blocks.

Both B0 and B2 are possibly removed by the removal of if & endif.
Same situation for if/endif. E.g., in the following example we'd remove
blocks B1 and B2, and then attempt to combine B0 and B3.

B0: ...
B1: (+f0) if(8)
B2: endif(8)
B3: ...

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>