[tuples] Cleanup some statement flags

["Diego Novillo" <dnovillo at google dot com>]

As we convert code to tuples, we find various bitflags that used to be
stored in random fields inside the tree.  In tuples, we have between 8
and 16 bits worth of space for various bit flags.

Flags that are common to every statement go in the 8-bit field 'flags'
in gimple_statement_base.  That's not a lot of room, so only the most
commonly used flags will end up there.

For flags that are only useful in certain statements, we have a bit
more freedom.  If the statement does not use subcodes, then there are
8 additional bits for setting flags.  This is the case of GIMPLE_ASM,
the 'volatile' flag for GIMPLE_ASM can live in the subcode field.

All this complication is not exposed to the user code, however.  All
these flags are get/set by inline functions in gimple.h.  So, we can
change where the flags live very easily.  I can see some of these
flags ending up implemented as pointer maps, hashtables or bitmaps.

This patch re-organizes the few flags we have defined for now so that
it's easier to add/remove flags as we go along.

Tested on x86_64.  Committed.


Diego.

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