fixup! Be somewhat clearer about upper bits in the codegen.

ykrt/src/compile/jitc_yk/codegen/x64/mod.rs

@@ -1650,6 +1650,9 @@ impl<'a> Assemble<'a> {
     }
 
     /// When we're about to make a call, we need to clear upper bits in registers.
+    ///
+    /// This function clears those bits in a manner that is compatible with the SysV ABI. Note:
+    /// we're a little bit OTT here, and we could do less work if we become more clever about it.
     fn clear_bits_for_call(&mut self, gp_cnstrs: &[RegConstraint<Rq>]) {
         for gp_cnstr in gp_cnstrs {
             match gp_cnstr {