Fix #4137: Keep nint in conditionals with mixed native int branches

When the branches of a conditional have different types, its type falls
back to the IL stack slot type, which is (U)IntPtr. IntroduceNativeIntTypeOnLocals
only retypes variables, so with native integers enabled the branches got
cast to IntPtr. Map (U)IntPtr to n(u)int the same way boxing does; pointer
types are unaffected.

ICSharpCode.Decompiler.Tests/TestCases/Pretty/NativeInts.cs

@@ -254,5 +254,10 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			// Explicit `unchecked` is necessary when casting oversized constant to nuint
 			return unchecked((nuint)9123123123123uL);
 		}
+
+		public nint ConditionalWithMixedBranchTypes(bool b, int i, nuint x, nint y)
+		{
+			return (b ? ((nint)i) : ((nint)x)) - y;
+		}
 	}
 }