freedreno/ir3: helpful iterator macros

freedreno/ir3: helpful iterator macros

I remembered that we are using c99.. which makes some sugary iterator
macros easier.  So introduce iterator macros to iterate all src
registers and all SSA src instructions.  The _n variants also return
the src #, since there are a handful of places that need this.

Signed-off-by: Rob Clark <robclark@freedesktop.org>