Add a valgrind suppression for the llvm::User leak

It's been around forever. I don't know what it is, but I need the compiler to
be valgrind clean for the istr transition so I'm suppressing it.

src/etc/x86.supp

@@ -349,3 +349,9 @@
    ...
 }
 
+{  
+   llvm-user-new-leak
+   Memcheck:Leak
+   fun:_Znwj
+   fun:_ZN4llvm4UsernwEjj
+}