This will prevent a regression in the number of inlined constants in
a later commit. Constructs like 4.000000 (0x48).w110 works just fine.

There is a small behavioral change. We would previously allow positive
and negative same-value contants to be produced, e.g.,
4.000000 (0x48).w-w__ and this would be later split into some extra
movs in the dataflow swizzle pass. We now explicitly check that the
final swizzle is valid while inlining. So there is a minor decrease
in inlined constants and in the total instructions.

total lits in shared programs: 4328 -> 4194 (-3.10%)
lits in affected programs: 554 -> 420 (-24.19%)
total instructions in shared programs: 155488 -> 155361 (-0.08%)
instructions in affected programs: 5707 -> 5580 (-2.23%)

Additonally, a fix for pair translation is needed since the constant
inlining can now produce swizzles like this: 4.000000 (0x48).w-0-0-_
so we have to teach pair translation to also ignore the sign for zero
swizzle.

Signed-off-by: Pavel Ondračka <pavel.ondracka@gmail.com>
Reviewed-by: Filip Gawin <filip@gawin.net>
Part-of: <!17560>