i965/vec4: add a scalarization pass for double-precision instructions

The hardware only supports 32-bit swizzles, which means that we can
only access directly channels XY of a DF making access to channels ZW
more difficult, specially considering the various regioning restrictions
imposed by the hardware. The combination of both things makes handling
ramdom swizzles on DF operands rather difficult, as there are many
combinations that can't be represented at all, at least not without
some work and some level of instruction splitting depending on the case.

Writemasks are 64-bit in general, however XY and ZW writemasks also work
in 32-bit, which means these writemasks can't be represented natively,
adding to the complexity.

For now, we decided to try and simplify things as much as possible to
avoid dealing with all this from the get go by adding a scalarization
pass that runs after the main optimization loop. By fully scalarizing
DF instructions in align16 we avoid most of the complexity introduced
by the aforementioned hardware restrictions and we have an easier path
to an initial fully functional version for the vector backend in Haswell
and IvyBridge.

Later, we can improve the implementation so we don't necessarily
scalarize everything, iteratively adding more complexity and building
on top of a framework that is already working. Curro drafted some ideas
for how this could be done here:
https://bugs.freedesktop.org/show_bug.cgi?id=92760#c82

v2:
  - Use a copy constructor for the scalar instructions so we copy all
    relevant instructions fields from the original instruction.

v3: Fix indention in one switch (Matt)

Reviewed-by: Matt Turner <[email protected]>

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