llvm::DenseMap is unsuitable with arbitrary integer keys

It needs to reserve some special key values that the caller
has to promise to never insert into the map.  For integers that's
-1 and -2... which we were using and then got some random data back.

src/asm_writing/rewriter.h

@@ -321,7 +321,7 @@ private:
         // Loads the constant into any register or if already in a register just return it
         assembler::Register loadConst(uint64_t val, Location otherThan = Location::any());
 
-        llvm::DenseMap<uint64_t, RewriterVar*> constToVar;
+        std::unordered_map<uint64_t, RewriterVar*> constToVar;
     };