irinterp: refine :nothrow only when it is not proved yet

This makes irinterp not override `:nothrow=true` that are assumed by
`Base.@assume_effects`.

base/compiler/abstractinterpretation.jl

@@ -1153,8 +1153,11 @@ function semi_concrete_eval_call(interp::AbstractInterpreter,
                 # that are newly resovled by irinterp
                 # state = InliningState(interp)
                 # ir = ssa_inlining_pass!(irsv.ir, state, propagate_inbounds(irsv))
-                new_effects = Effects(result.effects; nothrow)
-                return ConstCallResults(rt, SemiConcreteResult(mi, ir, new_effects), new_effects, mi)
+                effects = result.effects
+                if !is_nothrow(effects)
+                    effects = Effects(effects; nothrow)
+                end
+                return ConstCallResults(rt, SemiConcreteResult(mi, ir, effects), effects, mi)
             end
         end
     end

test/compiler/effects.jl

@@ -998,3 +998,16 @@ f50198() = (hf50198(Ref(:x)[]); nothing)
 f50311(x, s) = Symbol(s)
 g50311(x) = Val{f50311((1.0, x), "foo")}()
 @test fully_eliminated(g50311, Tuple{Float64})
+
+# irinterp should refine `:nothrow` information only if profitable
+Base.@assume_effects :nothrow function irinterp_nothrow_override(x, y)
+    z = sin(y)
+    if x
+        return "julia"
+    end
+    return z
+end
+@test Base.infer_effects((Float64,)) do y
+    isinf(y) && return zero(y)
+    irinterp_nothrow_override(true, y)
+end |> Core.Compiler.is_nothrow