[llvm] [DSE] Stop double-counting the dead slice offset in shortenAssignment (PR #216227)

[Orlando Cazalet-Hyams via llvm-commits]

================
@@ -16,22 +16,37 @@
 ;;   __builtin_memset(local2, 8, 16); // local2:  0-128  (0, 128)
 ;;   esc(local2);
 ;; }
+;; void shortenEndPartial() {
+;;   char local3[80];                      //         bits    frag
+;;   __builtin_memset(local3 + 8,  0, 8);  // local3:  64-128 (64, 96)
+;;   __builtin_memset(local3 + 12, 8, 4);  // local3:  96-128 ( 96, 32)
----------------
OCHyams wrote:

```suggestion
;;   __builtin_memset(local3 + 12, 8, 4);  // local3:  96-128 (96, 32)
```
nit

https://github.com/llvm/llvm-project/pull/216227