func.shader: add tests for undefined dual-source blend outputs

Open Rhys Perry requested to merge pendingchaos/crucible:dual_src_blend_undef_mrt into main

nir_opt_undef will eliminate outputs if undef is written to them. This can cause problems.

Adds func.shader.dualsrc_mrt0_undef and func.shader.dualsrc_mrt1_undef.

Merge request reports