While they used Cranelift IR itself (amongst others, not just LLVM) to show performance improvements (thus making it complementary and not a replacement) you raise a good point. Quite possible it is not as full-featured yet so perhaps in the future, if at all.
The TPDE-based back-end compiles 4.27x faster than Cranelift and 2.68x faster than Cranelift with its fast register allocator, but is 1.74x slower than Winch
The TPDE-based back-end compiles 4.27x faster than Cranelift and 2.68x faster than Cranelift with its fast register allocator, but is 1.74x slower than Winch