i965/vec4: keep original type when dealing with null registers

i965/vec4: keep original type when dealing with null registers

Keep the original type when dealing with null registers. Especially
because we do no want to introduce an implicit conversion between
types that could affect the conditional flags.

This affects especially when the original type is DF, and we are working
on Ivybridge/Baytrail.

v2 (Curro)
- Fix typo.
- Use retype() instead of applying the type directly.
- Remove unneeded retype.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>