nir/cf: Remove phi sources if needed in nir_handle_add_jump

If the block in which the jump is inserted is the predecessor of a phi
then we need to remove phi sources otherwise the phi may end up with
things improperly connected.  This fixes the following CTS test when
dEQP is run with SPIR-V optimization recipe 1:

dEQP-VK.glsl.functions.control_flow.return_in_nested_loop_vertex

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
(cherry picked from commit bfc89c66)