emit_x64: Make reg_alloc a local variable

reg_alloc contains state that is only valid on a per-block basis, so there
is no reason for it to be a member variable.

src/backend_x64/emit_x64.h

@@ -54,7 +54,7 @@ public:
 
 private:
     // Microinstruction emitters
-#define OPCODE(name, type, ...) void Emit##name(IR::Block& block, IR::Inst* inst);
+#define OPCODE(name, type, ...) void Emit##name(RegAlloc& reg_alloc, IR::Block& block, IR::Inst* inst);
 #include "frontend/ir/opcodes.inc"
 #undef OPCODE
 
@@ -87,9 +87,6 @@ private:
     // Global CPU information
     Xbyak::util::Cpu cpu_info;
 
-    // Per-block state
-    RegAlloc reg_alloc;
-
     // State
     BlockOfCode* code;
     UserCallbacks cb;