i965/fs: Allow constant propagation between different types

i965/fs: Allow constant propagation between different types

This will be needed for NIR because it is typeless and treats all constants
as uint32 values and reinterprets them when they are used later.  This
commit allows those values to be properly propagated.

Also, this helps some synmark shaders because it allows us to copy
propagate a 0x00000000UD into a 0.0F in a load_payload, which then lets us
combine 4 load_payloads.

instructions in affected programs:     2288 -> 2144 (-6.29%)

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