compact: Don't attempt to amend basic block 0 (#54566)

This allows insert_node_here! with reverse affinity at the start of the
first basic block, which is a bit of a corner case, but no reason for it
to be disallowed.

base/compiler/ssair/ir.jl

@@ -1018,7 +1018,8 @@ function did_just_finish_bb(compact)
  result_idx = compact.result_idx
  result_bbs = compact.cfg_transform.result_bbs
  (compact.active_result_bb == length(result_bbs) + 1) ||
- result_idx == first(result_bbs[compact.active_result_bb].stmts)
+ (result_idx == first(result_bbs[compact.active_result_bb].stmts) &&
+ compact.active_result_bb != 1)
 end
 
 function maybe_reopen_bb!(compact)

test/compiler/compact.jl

@@ -35,3 +35,17 @@ end
  verify_ir(new_ir)
  @test length(new_ir.cfg.blocks) == 1
 end
+
+# Test reverse affinity insert at start of compact
+@testset "IncrementalCompact reverse affinity insert" begin
+ ir = only(Base.code_ircode(foo_test_function, (Int,)))[1]
+ compact = IncrementalCompact(ir)
+ @test !Core.Compiler.did_just_finish_bb(compact)
+
+ insert_node_here!(compact, NewInstruction(ReturnNode(1), Union{}, ir[SSAValue(1)][:line]), true)
+ new_ir = finish(compact)
+ # TODO: Should IncrementalCompact be doing this internally?
+ empty!(new_ir.cfg.blocks[1].succs)
+ verify_ir(new_ir)
+ @test length(new_ir.cfg.blocks) == 1
+end