i965/vec4: Fix constant propagation across different types.

i965/vec4: Fix constant propagation across different types.

If the source type differs from the original type of the constant we
need to bit-cast it before propagating, otherwise the original type
information will be lost.  If the constant was a vector float there
isn't much we can do, because the result of bit-casting the component
values of a vector float cannot itself be represented as an immediate.

Reviewed-by: Matt Turner <mattst88@gmail.com>