Admit more dest addresses in block assign value-numbering.

Admit more dest addresses in block assign value-numbering.

Internal testing revealed an assertion when computing the value number
for block assignments: the VN framework expected any block assignments
to a tracked lclVar to have a destination address of the form
`(addr (lclVar))` or `(addr (lclFld))`. However, the check that it
uses to determine whether or not a block assignment targets a lclVar
also admits addresses formed by some combination of adds of constants
to these patterns (e.g. `(add (const 4) (add (addr lclVar) (const 4)))`.

This fix admits these additional patterns in value numbering by allowing
`ExtendPtrVN(GenTree*, GenTree*)` to supply `NotAField` to
`ExtendPtrVN(GenTree*, FieldSeqNode*)`.