optimize: Delete incoming unreachable edges from PhiNode

Incoming IR very rarely contains PhiNodes, but we do allow it to make things
easier on downstream packages like Diffractor that want to generate the same
code structures in both typed and untyped mode. However, this does of course
mean that once inference is finished, any PhiNodes in the original source
must be adjusted to maintain IRCode invariants. One particular important
invariant here is that the edges list in a PhiNode must match the
predecessor list, so in particular if a predecessor becomes unreachable
during inference, we must filter that edge before passing it on to the
optimizer.

base/compiler/optimize.jl

@@ -1104,6 +1104,19 @@ function convert_to_ircode(ci::CodeInfo, sv::OptimizationState)
                         code[i] = nothing
                     end
                 end
+            elseif isa(expr, PhiNode)
+                new_edges = Int32[]
+                new_vals = Any[]
+                for j = 1:length(expr.edges)
+                    (expr.edges[j] in sv.unreachable) && continue
+                    push!(new_edges, expr.edges[j])
+                    if isassigned(expr.values, j)
+                        push!(new_vals, expr.values[j])
+                    else
+                        resize!(new_vals, length(new_edges))
+                    end
+                end
+                code[i] = PhiNode(new_edges, new_vals)
             end
         end
     end

test/compiler/interpreter_exec.jl

@@ -110,3 +110,29 @@ let m = Meta.@lower 1 + 1
     @test :b === @eval $m
     @test isempty(current_exceptions())
 end
+
+# Test that things don't break if one branch of the frontend PhiNode becomes unreachable
+let m = Meta.@lower 1 + 1
+    @assert Meta.isexpr(m, :thunk)
+    src = m.args[1]::CodeInfo
+    src.code = Any[
+        # block 1
+        GlobalRef(@__MODULE__, :global_error_switch),
+        GotoIfNot(SSAValue(1), 4),
+        # block 2
+        Expr(:call, error, "This error is expected"),
+        # block 3
+        PhiNode(Int32[2, 3], Any[1, 2]),
+        ReturnNode(SSAValue(4))
+    ]
+    nstmts = length(src.code)
+    src.ssavaluetypes = nstmts
+    src.ssaflags = fill(UInt8(0x00), nstmts)
+    src.debuginfo = Core.DebugInfo(:none)
+    Core.Compiler.verify_ir(Core.Compiler.inflate_ir(src))
+    global global_error_switch = true
+    @test_throws ErrorException @eval $m
+    global global_error_switch = false
+    @test 1 === @eval $m
+    @test isempty(current_exceptions())
+end