JIT: Generalize hasFixedRetBuffReg for ARM64 (#100277)

Move a check from `CallArgs::GetCustomRegister` to `hasFixedRetBuffReg`.

Author: jakobbotsch

src/coreclr/jit/gentree.cpp

@@ -1806,11 +1806,7 @@ regNumber CallArgs::GetCustomRegister(Compiler* comp, CorInfoCallConvExtension c
         case WellKnownArg::RetBuffer:
             if (hasFixedRetBuffReg(cc))
             {
-                // Windows does not use fixed ret buff arg for instance calls, but does otherwise.
-                if (!TargetOS::IsWindows || !callConvIsInstanceMethodCallConv(cc))
-                {
-                    return theFixedRetBuffReg(cc);
-                }
+                return theFixedRetBuffReg(cc);
             }
 
             break;

src/coreclr/jit/target.h

@@ -422,7 +422,8 @@ inline bool genIsValidDoubleReg(regNumber reg)
 inline bool hasFixedRetBuffReg(CorInfoCallConvExtension callConv)
 {
 #if defined(TARGET_ARM64)
-    return true;
+    // Windows does not use fixed ret buff arg for instance calls, but does otherwise.
+    return !TargetOS::IsWindows || !callConvIsInstanceMethodCallConv(callConv);
 #elif defined(TARGET_AMD64) && defined(SWIFT_SUPPORT)
     return callConv == CorInfoCallConvExtension::Swift;
 #else