Delete GenTreePtr. (#16027)

Delete GenTreePtr. (#16027)

* jit sources: Each local pointer variable must be declared on its own line.

Implement https://github.com/dotnet/coreclr/blob/master/Documentation/coding-guidelines/clr-jit-coding-conventions.md#101-pointer-declarations
Each local pointer variable must be declared on its own line.

* add constGenTreePtr

* delete GenTreePtr

* delete constGenTreePtr

* fix arm