It is not critical for register assignment -- in fact, SSA makes register assignment more difficult (see the swap problem; the lost copy problem). Lifetime ana

It is not critical for register assignment -- in fact, SSA makes register assignment more difficult (see the swap problem; the lost copy problem). Lifetime analysis is important for register assignment, and SSA can make lifetime analysis easier, but plenty of non-SSA compilers (lower-tier JIT compilers often do not use SSA because SSA is heavyweight) are able to register allocate just fine without it.