fix releasing in allocator

fell into hash new trap, which reuses the object you give it. not good for mutable objects like the array.
also previous logic was broken in terms of machine vs ssa names

test/risc/test_standard_allocator1.rb

@@ -13,7 +13,7 @@ module Risc
     end
     def test_allocate_runs
       assert_nil @allocator.allocate_regs
-      assert_equal 10 , @allocator.used_regs.length
+      assert_equal 0 , @allocator.used_regs.length
     end
     def test_live_length
       live = @allocator.walk_and_mark(@compiler.risc_instructions)