Use &mut Bx more.

For the next commit, `FunctionCx::codegen_*_terminator` need to take a
`&mut Bx` instead of consuming a `Bx`. This triggers a cascade of
similar changes across multiple functions. The resulting code is more
concise and replaces many `&mut bx` expressions with `bx`.

compiler/rustc_codegen_ssa/src/traits/builder.rs

@@ -151,11 +151,11 @@ pub trait BuilderMethods<'a, 'tcx>:
 
     /// Called for Rvalue::Repeat when the elem is neither a ZST nor optimizable using memset.
     fn write_operand_repeatedly(
-        self,
+        &mut self,
         elem: OperandRef<'tcx, Self::Value>,
         count: u64,
         dest: PlaceRef<'tcx, Self::Value>,
-    ) -> Self;
+    );
 
     fn range_metadata(&mut self, load: Self::Value, range: WrappingRange);
     fn nonnull_metadata(&mut self, load: Self::Value);