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 nilness output by providing position data
with the message.

Updates golang/go#31008

Change-Id: I664c8668bc74207e770657e21129d20fd70e7af6
GitHub-Last-Rev: 75831d2
GitHub-Pull-Request: #190
Reviewed-on: https://go-review.googlesource.com/c/tools/+/208599
Run-TryBot: Robert Findley <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: vovapi

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: