[SimplifyCFG] Fix crash when there is unreachable large index (#88616)

The large case index out of scope is dead code, but it is still be
created for TableContents in SwitchLookupTable::SwitchLookupTable,
so make sure the table size after growing should not get smaller.

Fix #88607

Author: vfdff

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -6783,9 +6783,11 @@ static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
           return SwitchLookupTable::WouldFitInRegister(
               DL, UpperBound, KV.second /* ResultType */);
         })) {
+      // There may be some case index larger than the UpperBound (unreachable
+      // case), so make sure the table size does not get smaller.
+      TableSize = std::max(UpperBound, TableSize);
       // The default branch is unreachable after we enlarge the lookup table.
       // Adjust DefaultIsReachable to reuse code path.
-      TableSize = UpperBound;
       DefaultIsReachable = false;
     }
   }

llvm/test/Transforms/SimplifyCFG/switch_mask.ll

@@ -214,3 +214,28 @@ lor.end:                                          ; preds = %default, %for.end,
   %retval.0.i.i = phi i32 [ 1, %default ], [ 0, %for.end ], [ 0, %for.end ], [ 0, %for.end ]
   ret void
 }
+
+define i1 @pr88607() {
+; CHECK-LABEL: @pr88607(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[COND:%.*]] = select i1 false, i32 4, i32 1
+; CHECK-NEXT:    [[SPEC_SELECT:%.*]] = select i1 false, i32 2, i32 [[COND]]
+; CHECK-NEXT:    ret i1 false
+;
+entry:
+  %cond = select i1 false, i32 4, i32 1
+  %spec.select = select i1 false, i32 2, i32 %cond
+  switch i32 %spec.select, label %lor.rhs [
+  i32 0, label %exit
+  i32 5, label %exit ; unreachable large case index
+  i32 1, label %exit
+  ]
+
+lor.rhs:                                        ; preds = %entry
+  br label %exit
+
+exit: ; preds = %lor.rhs, %entry, %entry, %entry, %entry
+  %res.ph = phi i1 [ false, %entry ], [ false, %lor.rhs ], [ false, %entry ], [ false, %entry ]
+  ret i1 %res.ph
+}
+