fix rowserrcheck reports false positive

[jingyugao]

Fixes #943

This PR fix that rowserrcheck reports false positive when handling phi instruction.

For example

func badcase() {
	var db *sql.DB
	var rows *sql.Rows
	if rand.Float64() < 0.5 {
		rows, _ = db.Query("select 1")
	}
	if err := rows.Err(); err != nil {
		panic(err)
	}
}

which ssa is as below:

func badcase():
0:                                                                entry P:0 S:2
	t0 = math/rand.Float64()                                        float64
	t1 = t0 < 0.5:float64                                              bool
	if t1 goto 1 else 2
1:                                                              if.then P:1 S:1
	t2 = (*database/sql.DB).Query(nil:*database/sql.DB, "select 1":string, nil:[]interface{}...) (*database/sql.Rows, error)
	t3 = extract t2 #0                                   *database/sql.Rows
	t4 = extract t2 #1                                                error
	jump 2
2:                                                              if.done P:2 S:2
	// t5'Err is checked,but t5 is not t3's referrer. t5 is phi(...,t3)'referrer.
        t5 = phi [0: nil:*database/sql.Rows, 1: t3] #rows    *database/sql.Rows
	t6 = (*database/sql.Rows).Err(t5)                                 error
	t7 = t6 != nil:error                                               bool
	if t7 goto 3 else 4
3:                                                              if.then P:1 S:0
	t8 = change interface interface{} <- error (t6)             interface{}
	panic t8
4:                                                              if.done P:1 S:0
	return

if we move the assignment statement out of the if block.

func passcase() {
	var db *sql.DB
	var rows *sql.Rows
	if rand.Float64() < 0.5 {
		rows, _ = db.Query("select 1")
	}
	if err := rows.Err(); err != nil {
		panic(err)
	}
}

ths ssa is below

0:                                                                entry P:0 S:2
	t0 = math/rand.Float64()                                        float64
	t1 = t0 < 0.5:float64                                              bool
	t2 = (*database/sql.DB).Query(nil:*database/sql.DB, "select 1":string, nil:[]interface{}...) (*database/sql.Rows, error)
	t3 = extract t2 #0                                   *database/sql.Rows
	t4 = extract t2 #1                                                error
        // here t3 is t2'referrer's referrer
	t5 = (*database/sql.Rows).Err(t3)                                 error
	t6 = t5 != nil:error                                               bool
	if t6 goto 1 else 2
1:                                                              if.then P:1 S:0
	t7 = change interface interface{} <- error (t5)             interface{}
	panic t7
2:                                                              if.done P:1 S:0
	return

[CLAassistant]

All committers have signed the CLA.

[ldez]

@jingyugao there is an issue with the authorship of your commits, could you recreate your commits with a valid author?

also could you merge jingyugao/rowserrcheck#13

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