r300/compiler: Standardize the number of bits used by swizzle fields

r300/compiler: Standardize the number of bits used by swizzle fields

Swizzles are now defined everywhere as a field with 12 bits that contains
4 channels worth of meaningful information.  Any channel that is unused is
set to RC_SWIZZLE_UNUSED.  This change is necessary because rgb instructions
and alpha instructions were initializing channels that would never be used
(channel 3 for rgb and channels 1-3 for alpha) with 0 (aka RC_SWIZZLE_X).
This made it impossible to use generic helper functions for swizzles,
because sometimes a channel value of 0 meant unused and other times it
meant RC_SWIZZLE_X.

All hacks that tried to guess how many channels were relevant have
also been removed.