pp_add: fix UB in a right shift

Use UVINTSIZE to calculate the number of bits of a UVINT variable.

The invalid number of bits in the right shift resulted in a poisoned
variable. A later branch depending on that variable becomes UB when
-branch-on-poison-as-ub is enabled in LLVM, which was turned on by
default in LLVM 15 [1].

[1] llvm/llvm-project@03aceab

Pointed out by:	@jrtc27

pp_hot.c

@@ -1423,8 +1423,8 @@ PP(pp_add)
             il = SvIVX(svl);
             ir = SvIVX(svr);
           do_iv:
-            topl = ((UV)il) >> (UVSIZE * 8 - 2);
-            topr = ((UV)ir) >> (UVSIZE * 8 - 2);
+            topl = ((UV)il) >> (UVINTSIZE * 8 - 2);
+            topr = ((UV)ir) >> (UVINTSIZE * 8 - 2);
 
             /* if both are in a range that can't under/overflow, do a
              * simple integer add: if the top of both numbers