spirv/cfg: Make the builder fully capable for both walks

spirv/cfg: Make the builder fully capable for both walks

We were only initializing vtn_builder::func for the pre-walk where we
build the CFG.  We were only initializing the nir_builder for the later
walk through the instructions even though were were setting b->cursor
for the pre-walk.  Let's set both both places so that everything is
consistent.  This useful because we handle OpFunctionParameter in the
pre-walk and we're going to need to be able to emit instructions.

Acked-by: Rob Clark <robdclark@gmail.com>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>