Improve effects for Base.fieldindex (#50199)

Split out the error path into a function with separate effects assumptions,
so that constant propagation on `err` can conclude that the `err=false` case
does not throw. Fixes #50198.

base/reflection.jl

@@ -820,9 +820,19 @@ julia> Base.fieldindex(Foo, :z, false)
 ```
 """
 function fieldindex(T::DataType, name::Symbol, err::Bool=true)
+    return err ? _fieldindex_maythrow(T, name) : _fieldindex_nothrow(T, name)
+end
+
+function _fieldindex_maythrow(T::DataType, name::Symbol)
     @_foldable_meta
     @noinline
-    return Int(ccall(:jl_field_index, Cint, (Any, Any, Cint), T, name, err)+1)
+    return Int(ccall(:jl_field_index, Cint, (Any, Any, Cint), T, name, true)+1)
+end
+
+function _fieldindex_nothrow(T::DataType, name::Symbol)
+    @_total_meta
+    @noinline
+    return Int(ccall(:jl_field_index, Cint, (Any, Any, Cint), T, name, false)+1)
 end
 
 function fieldindex(t::UnionAll, name::Symbol, err::Bool=true)

test/compiler/effects.jl

@@ -988,3 +988,8 @@ isassigned_effects(s) = isassigned(Ref(s))
 @test fully_eliminated(; retval=true) do
     isassigned_effects(:foo)
 end
+
+# Effects of Base.hasfield (#50198)
+hf50198(s) = hasfield(typeof((;x=1, y=2)), s)
+f50198() = (hf50198(Ref(:x)[]); nothing)
+@test fully_eliminated(f50198)