spirv: Move cursor before calling vtn_ssa_value() in phi 2nd pass.

vtn_ssa_value() can produce variable loads, and the cursor might
be after a return statement, causing nir_builder assert failures
about not inserting instructions after a jump.

This fixes:
dEQP-VK.spirv_assembly.instruction.graphics.barrier.in_if
dEQP-VK.spirv_assembly.instruction.graphics.barrier.in_switch

Cc: "13.0 12.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>