The handling of write masks and swizzles for 64 bit data types is
currently irregular: write masks are always 64 bit, while swizzles
are usually 32 bit, except for SSA registers with are 64 bit.
With this change we always use 64 bit swizzles, in order to make
the situation less surprising and make it easier to convert
registers between SSA and TEMP.
64 bit swizzles are always required to have X in their last two
components.