nir: Add some helpers for chasing SSA values properly

There are various cases in which we want to chase SSA values through ALU
ops ranging from hand-written optimizations to back-end translation
code.  In all these cases, it can be very tricky to do properly because
of swizzles.  This set of helpers lets you easily work with a single
component of an SSA def and chase through ALU ops safely.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
(cherry picked from commit 8f7405ed)
[Juan A. Suarez: resolve trivial conflicts]
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>

Conflicts:
	src/compiler/nir/nir.h