Arm64Gen: Move constant and make constexpr

Namespace-scope variable was only used in one function so move it there

Source/Core/Common/Arm64Emitter.cpp

@@ -26,8 +26,6 @@ namespace Arm64Gen
 {
 namespace
 {
-const int kWRegSizeInBits = 32;
-
 uint64_t LargestPowerOf2Divisor(uint64_t value)
 {
   return value & -(int64_t)value;
@@ -88,6 +86,8 @@ std::optional<std::tuple<u32, u32, u32>> IsImmLogical(u64 value, u32 width)
     value = ~value;
   }
 
+  constexpr int kWRegSizeInBits = 32;
+
   if (width == kWRegSizeInBits)
   {
     // To handle 32-bit logical immediates, the very easiest thing is to repeat