[tree-ssa] Minor cleanup for dominator walker

[Zdenek Dvorak]

Hello,

> This minor cleanup has been on my todo list for awhile.
> 
> Basically this changes the last argument in the dominator walker callbacks
> to be the parent block in the dominator tree rather than the last statement
> in the parent block.

would not it be easier just not to pass this argument at all and just
use get_immediate_dominator in the places where it is needed?

Zdenek