i965: Add lowering pass to fold offset into unnormalized coords

i965: Add lowering pass to fold offset into unnormalized coords

It turns out that nonzero offsets with gsampler2DRect don't work -- they
just return garbage. Work around this by folding the offset into the
coord.

Done as an IR pass rather than yet another hack in the visitors because
it's clear what's going on this way. Can possibly reuse this to replace
the existing txf coord+offset hacks.

V2: Use ir_builder

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>