ConstantExpressionRunner cannot handle `drop` case

[HerrCai0907]

when I use ConstantExpressionRunner in FlagValues::DEFAULT mode to calculate code like

(block $label$4 (result i32)
 (drop
  (local.tee $7
   (local.get $8)
  )
 )
 (i32.const 0)
)

current implement will return NONCONSTANT_FLOW instead of i32.const 0 if local $8 is unknown.