DSPCore: Convert DSP stack register enum into an enum class

Makes it more self-documenting which stack is being loaded or stored to,
as C, D, and magic numbers are extremely vague. It also enforces a
strongly-typed API instead of accepting arbitrary integral values.

It also adds the two other missing stack register names -- loop address
and loop counter.

Source/Core/Core/DSP/DSPStacks.cpp

@@ -3,38 +3,44 @@
 // Licensed under GPLv2+
 // Refer to the license.txt file included.
 
-#include "Common/CommonTypes.h"
-
-#include "Core/DSP/DSPCore.h"
 #include "Core/DSP/DSPStacks.h"
 
+#include <cstddef>
+
+#include "Common/CommonTypes.h"
+#include "Core/DSP/DSPCore.h"
+
 // Stacks. The stacks are outside the DSP RAM, in dedicated hardware.
 namespace DSP
 {
-static void dsp_reg_stack_push(int stack_reg)
+static void dsp_reg_stack_push(size_t stack_reg)
 {
   g_dsp.reg_stack_ptr[stack_reg]++;
   g_dsp.reg_stack_ptr[stack_reg] &= DSP_STACK_MASK;
   g_dsp.reg_stack[stack_reg][g_dsp.reg_stack_ptr[stack_reg]] = g_dsp.r.st[stack_reg];
 }
 
-static void dsp_reg_stack_pop(int stack_reg)
+static void dsp_reg_stack_pop(size_t stack_reg)
 {
   g_dsp.r.st[stack_reg] = g_dsp.reg_stack[stack_reg][g_dsp.reg_stack_ptr[stack_reg]];
   g_dsp.reg_stack_ptr[stack_reg]--;
   g_dsp.reg_stack_ptr[stack_reg] &= DSP_STACK_MASK;
 }
 
-void dsp_reg_store_stack(int stack_reg, u16 val)
+void dsp_reg_store_stack(StackRegister stack_reg, u16 val)
 {
-  dsp_reg_stack_push(stack_reg);
-  g_dsp.r.st[stack_reg] = val;
+  const auto reg_index = static_cast<size_t>(stack_reg);
+
+  dsp_reg_stack_push(reg_index);
+  g_dsp.r.st[reg_index] = val;
 }
 
-u16 dsp_reg_load_stack(int stack_reg)
+u16 dsp_reg_load_stack(StackRegister stack_reg)
 {
-  u16 val = g_dsp.r.st[stack_reg];
-  dsp_reg_stack_pop(stack_reg);
+  const auto reg_index = static_cast<size_t>(stack_reg);
+
+  const u16 val = g_dsp.r.st[reg_index];
+  dsp_reg_stack_pop(reg_index);
   return val;
 }
 }  // namespace DSP