nir/vars_to_ssa: Add phi sources for unreachable predecessors
nir/vars_to_ssa: Add phi sources for unreachable predecessors

It is possible to end up with unreachable blocks if, for instance, you have
an "if (...) { break; } else { continue; } unreachable()".  In this case,
the unreachable block does not show up in the dominance tree so it never
gets visited.  Instead, we go and visit all of those in follow-on pass.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler