Delete OldStyleClearD. (#14874)
* Delete OldStyleClearD from flowgraph
BlockSetOps::Assign requires sets to have the same size, there is no an
additional risk to use ClearD then.
* delete OldStyleClearD from regalloc
regalloc doesn't create new local variables and doesn't change epoch.
* Delete OldStyleClearD from copyprop
CopyProp doesn't create new vars.
Also `VarSetOps::Assign(this, compCurLife, block->bbLiveIn); ` before
the loop requires epoch to be the same.
* Delete OldStyleClearD from assertionpop
Assertion prop doesn't change epoch.
* Delete OldStyleClearD from the emmit
Because it doesn't create new local vars.
* Delete declarations