i965/vec4: add double/float conversion pseudo-opcodes

These need to be emitted as align1 MOV's, since they need to have a
stride of 2 on the float register (whether src or dest) so that data
from another thread doesn't cross the middle of a SIMD8 register.

v2 (Iago):
- The float-to-double needs to align 32-bit data to 64-bit before doing the
conversion. This was doable in align16 when we tried to use an execsize
of 4, but with an execsize of 8 we would need another align1 opcode to do
that (since we need data to cross the middle of a SIMD register). Just
making the opcode handle this internally seems more practical that adding
another opcode just for this purpose and having the caller know about this
before converting.
- The double-to-float conversion produces 32-bit elements aligned to 64-bit
so we make the opcode re-pack the result to 32-bit and fit in one register,
as expected by SIMD4x2 operation. This still requires that callers reserve
two registers for the float data destination because we need to produce
64-bit aligned data first, and repack it later on the same destination
register, but it saves the need for a re-pack opcode only to achieve this
making the operation complete in a single opcode. Hopefully that is worth
the weirdness of the double register allocation...

Signed-off-by: Connor Abbott <[email protected]>
Signed-off-by: Iago Toral Quiroga <[email protected]>

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

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