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.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965: Generalize coord+offset lowering pass for ir_txf

ir_txf expects an ivec* coordinate, and may be larger than ivec2;
shuffle things around so that this will work.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965: Remove ir_txf coord+offset special case in visitors

Just let it be handled by the lowering pass.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>