go/ssa: add position data in indexed range loops

Indexed range loops are lacking position data
in their SSA representation, so add the position data
into an index operation

This improves the problem described in golang/go#31008
by providing position data with the nilness message

go/ssa/builder.go

@@ -1746,6 +1746,7 @@ func (b *builder) rangeIndexed(fn *Function, x Value, tv types.Type, pos token.P
 				Index: k,
 			}
 			instr.setType(t.Elem())
+			instr.setPos(x.Pos())
 			v = fn.emit(instr)
 
 		case *types.Pointer: // *array
@@ -1754,6 +1755,7 @@ func (b *builder) rangeIndexed(fn *Function, x Value, tv types.Type, pos token.P
 				Index: k,
 			}
 			instr.setType(types.NewPointer(t.Elem().Underlying().(*types.Array).Elem()))
+			instr.setPos(x.Pos())
 			v = emitLoad(fn, fn.emit(instr))
 
 		case *types.Slice:
@@ -1762,6 +1764,7 @@ func (b *builder) rangeIndexed(fn *Function, x Value, tv types.Type, pos token.P
 				Index: k,
 			}
 			instr.setType(types.NewPointer(t.Elem()))
+			instr.setPos(x.Pos())
 			v = emitLoad(fn, fn.emit(instr))
 
 		default: