Some VN refactoring (#61034)

* Remove "block" from VNApplySelectorsAssign

It was unused and all the callers passed the current block anyway.

Also some renaming to make Assign and AssignTypeCoerce methods consistent.

* Improve the readability of VNForMapStore

By naming parameters properly.

* Improve the readability of VNForMapSelect

By naming parameters properly.

* Pull ASG numbering out of fgValueNumberTree

In my upcoming substantive changes to this code the unreasonable level
of nesting started to significantly degrade the readability of code.

So, move it out of the main numbering function. The result is still
pretty huge and could be split up further, but this will do for now.

* Refactor VNApplySelectors

Make it use a loop instead of recursion.

Rename parameters to match the "Assign" variants.

Add standard header comments.

Author: SingleAccretion

src/coreclr/jit/compiler.h

@@ -5465,6 +5465,8 @@ class Compiler
     // assignment.)
     void fgValueNumberTree(GenTree* tree);
 
+    void fgValueNumberAssignment(GenTreeOp* tree);
+
     // Does value-numbering for a block assignment.
     void fgValueNumberBlockAssignment(GenTree* tree);