Dedicate a throwaway register.

src/chalk/codegen.cr

@@ -90,10 +90,8 @@ module Chalk
           generate! tree, entry.function, table, target, free
         when Trees::TreeBlock
           table = Table.new(table)
-          throwaway = free
-          free += 1
           tree.children.each do |child|
-            free += generate! child, table, throwaway, free
+            free += generate! child, table, THROWAWAY_REG, free
           end
         when Trees::TreeVar
           entry = table[tree.name]?

src/chalk/emitter.cr

@@ -4,6 +4,8 @@ module Chalk
     RETURN_REG = 14
     # The register into which the "stack pointer" is stored.
     STACK_REG  = 13
+    # Register used for throwing away values.
+    THROWAWAY_REG = 12
 
     # Module to emit instructions and store
     # them into an existing array.