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.
There are only three cases, and while the code is longer it is also
hopefully easier to read. Moreover, an error message is casted if
we're doing something unexpected.
PHI nodes must be fixed up after this pass, because the block references
might have become broken. For simplicitly this is not handled yet.
The goal for this pass is to make the CFG structurizer simpler, because
only conditional and unconditional branches must be supported.
Eventually this limitation might be lifted if there is advantage in
doing so.
