checker: improve pos for if expr

[Delta456]

Improves position for If-Expr conditions

Before

code.v:5:14: error: `if` expression requires an expression as the last statement of every branch
    3 |     mut start := 0
    4 | 
    5 |     start += if val < 20 { start += 1  } else { start }
      |              ~~~~~~~~~~~
    6 |     dump(start)
    7 | }

After

vlib/v/checker/tests/if_expr_last_branch_stmt_err.vv:5:30: error: `if` expression requires an expression as the last statement of every branch
   3 |     mut start := 0
   4 | 
   5 |     start = if val < 20 { start += 1 } else { start }
     |                                 ~~
   6 |     dump(start)
   7 | }

[spytheman]

Excellent work.