added a test

Author: yhls

test/compiler/ssair.jl

@@ -96,3 +96,19 @@ let
         Meta.isexpr(ex, :meta)
     end
 end
+
+# PR #32145
+# Make sure IncrementalCompact can handle blocks with predecessors of index 0
+# while removing blocks with no predecessors.
+let cfg = CFG(BasicBlock[
+    make_bb([]        , [2, 4]),
+    make_bb([1]       , [4, 5]),
+    make_bb([]        , [4]   ), # should be removed
+    make_bb([0, 1, 2] , [5]   ), # 0 predecessor should be preserved
+    make_bb([2, 3]    , []    ),
+], Int[])
+    code = Compiler.IRCode(
+        [], [], Int32[], UInt8[], cfg, LineInfoNode[], [], [], [])
+    compact = Compiler.IncrementalCompact(code, true)
+    @test length(compact.result_bbs) == 4 && 0 in compact.result_bbs[3].preds
+end