Delete impCheckForNullPointer (#61576)

Delete impCheckForNullPointer (#61576)

The comment above the method mentions "many problems" with leaving
null pointers around, but it is unclear what kind of problems. I can
only speculate those were the problems in legacy codegen which "could
not handle constant op1".

It also mentions that "we cannot even fold (null+offset)", which is
incorrect: "gtFoldExprConst" does in fact fold such expressions to
zero byrefs. It is also the case that spilling the null into a local
affects little as local assertion propagation happily propagates the
null back into its original place.

There was also a little bug associated with the method that got fixed:
morph was trying to use it, and in the process created uses of a local
that was not initialized, since the statement list used by the method
is the importer's one, invalid in morph.