i965/nir/vec4: Add get_nir_dst() and get_nir_src() methods

i965/nir/vec4: Add get_nir_dst() and get_nir_src() methods

These methods are essential for the implementation of the NIR->vec4 pass. They
work similar to their fs_nir counter-parts.

When processing instructions, these methods are invoked to resolve the
brw registers (source or destination) corresponding to the NIR sources
or destination. It uses the map of NIR register index to brw register for
all registers locally allocated in a block.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>