i965/fs: Move texel offset handling to visit(ir_texture *).

i965/fs: Move texel offset handling to visit(ir_texture *).

This moves the handling of non-constant texel offset subexpression trees
to the place where we visit other such subtrees.  It also removes some
uses of ir->offset in emit_texture_gen7, which will be useful when we
write the backend for our new upcoming IR.

Based on a patch by Connor Abbott.

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