glsl: Add methods to copy parts of one ir_constant into another.

glsl: Add methods to copy parts of one ir_constant into another.

- copy_masked_offset copies part of a constant into another,
  assign-like.

- copy_offset copies a constant into (a subset of) another,
  funcall-return like.

These methods are to be used to trace through assignments and function
calls when computing a constant expression.

Signed-off-by: Olivier Galibert <galibert@pobox.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net> [v1]