[GISel] #undef macros when they are no longer needed. NFC (#117652)

These macros are created inside a function. They should be undefined
before the end of the function.
diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index 321760e..abfca50 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -499,6 +499,8 @@
     RTLIBCASE(LLRINT_F);
   }
   llvm_unreachable("Unknown libcall function");
+#undef RTLIBCASE_INT
+#undef RTLIBCASE
 }
 
 /// True if an instruction is in tail position in its caller. Intended for