[torque] fix inaccurate UnsafeCast in array-slice.tq

Bug: chromium:1039112
Change-Id: Ia9a6a4f9c3782852bcb83dd334f5dba0c8cd74a5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2011823
Commit-Queue: Joshua Litt <joshualitt@chromium.org>
Reviewed-by: Joshua Litt <joshualitt@chromium.org>
Auto-Submit: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65893}

src/builtins/array-slice.tq

@@ -65,7 +65,7 @@ namespace array {
     for (let current: Smi = start; current < to; ++current) {
       const e: Object =
           sloppyElements.objects[current + kSloppyArgumentsParameterMapStart];
-      const newElement: JSAny = UnsafeCast<JSAny>(
+      const newElement = UnsafeCast<(JSAny | TheHole)>(
           e != TheHole ? argumentsContext[UnsafeCast<Smi>(e)] :
                          unmappedElements.objects[current]);
       // It is safe to skip the write barrier here because resultElements was