[llvm] [SimplifyCFG] Fix the compile crash for invalid upper bound value (PR #71351)

[via llvm-commits]

================
@@ -107,3 +105,103 @@ lor.end:
   %0 = phi i8 [ 15, %sw.bb0 ], [ 6, %sw.bb1 ], [ 7, %sw.bb2 ], [ 0, %default ]
   ret i8 %0
 }
+
+; Negative test: The default branch is unreachable, also it has no result.
+define i1 @switch_lookup_with_small_i1_default_unreachable(i32 %x) {
+; CHECK-LABEL: @switch_lookup_with_small_i1_default_unreachable(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[AND:%.*]] = and i32 [[X:%.*]], 15
+; CHECK-NEXT:    ret i1 false
+;
+entry:
+  %and = and i32 %x, 15
+  switch i32 %and, label %default [
+  i32 4, label %phi.end
+  i32 2, label %phi.end
+  i32 10, label %phi.end
+  i32 9, label %phi.end
+  i32 1, label %sw.bb1.i
+  i32 3, label %sw.bb1.i
+  i32 5, label %sw.bb1.i
+  i32 0, label %sw.bb1.i
+  i32 6, label %sw.bb1.i
+  i32 7, label %sw.bb1.i
+  i32 8, label %sw.bb1.i
+  ]
+
+sw.bb1.i:                                     ; preds = %entry, %entry, %entry, %entry, %entry, %entry, %entry
+  br label %phi.end
+
+default:                                      ; preds = %entry
+  unreachable
+
+phi.end: ; preds = %sw.bb1.i, %entry, %entry, %entry, %entry
+  %retval = phi i1 [ false, %sw.bb1.i ], [ false, %entry ], [ false, %entry ], [ false, %entry ], [ false, %entry ]
+  ret i1 %retval
+}
+
+; Negative test: The result in default reachable, but its value is not const.
+define i1 @switch_lookup_with_small_i1_default_nonconst(i64 %x) {
+; CHECK-LABEL: @switch_lookup_with_small_i1_default_nonconst(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[AND:%.*]] = and i64 [[X:%.*]], 15
+; CHECK-NEXT:    switch i64 [[AND]], label [[DEFAULT:%.*]] [
+; CHECK-NEXT:      i64 10, label [[LOR_END:%.*]]
+; CHECK-NEXT:      i64 1, label [[LOR_END]]
+; CHECK-NEXT:      i64 2, label [[LOR_END]]
+; CHECK-NEXT:    ]
+; CHECK:       default:
+; CHECK-NEXT:    [[CALL:%.*]] = tail call i1 @foo()
+; CHECK-NEXT:    br label [[LOR_END]]
+; CHECK:       lor.end:
+; CHECK-NEXT:    [[TMP0:%.*]] = phi i1 [ true, [[ENTRY:%.*]] ], [ [[CALL]], [[DEFAULT]] ], [ true, [[ENTRY]] ], [ true, [[ENTRY]] ]
+; CHECK-NEXT:    ret i1 [[TMP0]]
+;
+entry:
+  %and = and i64 %x, 15
+  switch i64 %and, label %default [
+  i64 10, label %lor.end
+  i64 1, label %lor.end
+  i64 2, label %lor.end
+  ]
+
+default:                                          ; preds = %entry
+  %call = tail call i1 @foo()
+  br label %lor.end
+
+lor.end:                                          ; preds = %entry, %entry, %entry, %default
+  %0 = phi i1 [ true, %entry ], [ %call, %default ], [ true, %entry ], [ true, %entry ]
+  ret i1 %0
+}
+
+; Negative test: The upper bound index of switch is swapped.
+define void @switch_lookup_with_nonconst_range(i32 %x, i1 %cond) {
+; CHECK-LABEL: @switch_lookup_with_nonconst_range(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_PREHEADER:%.*]]
+; CHECK:       for.preheader:
+; CHECK-NEXT:    br i1 [[COND:%.*]], label [[FOR_PREHEADER]], label [[LOR_END:%.*]]
+; CHECK:       lor.end:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %for.preheader
+
+for.preheader:                                    ; preds = %for.preheader, %entry
+  %add = add nuw i32 %x, 1                        ; the UpperBound is unconfirmed
+  br i1 %cond, label %for.preheader, label %for.end
+
+for.end:                                          ; preds = %for.preheader
+  switch i32 %add, label %default [
+  i32 0, label %lor.end
+  i32 1, label %lor.end
+  i32 5, label %lor.end
+  ]
+
+default:                                          ; preds = %for.end
+  br label %lor.end
+
+lor.end:                                          ; preds = %default, %for.end, %for.end, %for.end
+  %retval.0.i.i = phi i32 [ 0, %default ], [ 0, %for.end ], [ 0, %for.end ], [ 0, %for.end ]
----------------
vfdff wrote:

Yes, It can still reproduce this issue with different value for phi nodes, so I'll update it, thanks.
```
%retval.0.i.i = phi i32 [ 1, %default ], [ 0, %for.end ], [ 0, %for.end ], [ 0, %for.end ]
```

https://github.com/llvm/llvm-project/pull/71351