Stop using LIST nodes for PHI operand lists (dotnet/coreclr#20266)

Stop using LIST nodes for PHI operand lists (dotnet/coreclr#20266)

* Stop using LIST nodes for PHI operand lists

* Speed up PHI creation

Calling gtSetStmtInfo/fgSetStmtSeq every time an arg is added is rather expensive and not really needed. Costs are always 0 and the shape of the PHI tree is always the same.

Removal of GT_LIST nodes makes it easier to update the linear order incrementally when a GT_PHI_ARG node is added.

* Cleanup PHI value numbering

This replaces the manual traversal of PHI's list of uses with a range-based for loop and cleans up a bunch of convoluted code.

* Remove redundant AsPhi

* Remove list pointer union

* Update PHI equality check

* Add GenTreePhi comment

* Rename op to node

Commit migrated from https://github.com/dotnet/coreclr/commit/501c1e3629bbe89cbcec2ee8057ec111a88bdbc8