aco/cssa: reset equal_anc_out if merging fails
try_merge_merge_set()
expects equal_anc_out
to be Temp()
in the beginning, so we need to reset it in case it's used again.
Fixes compilation of metro_exodus/163b3b895730d37b with VK_PIPELINE_CREATE_2_DISABLE_OPTIMIZATION_BIT_KHR
.