i965/compiler: clean up nir_intrinsic_load_input for vertex shaders

i965/compiler: clean up nir_intrinsic_load_input for vertex shaders

This code to re-set the type of the source and destination is not
necessary since we never manipulate the types. Looks like a
left over from a time where we had to retype to float temporarily
to handle 64-bit inputs.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>