Continue cleaning GenTreeStmt. (dotnet/coreclr#26668)

Continue cleaning GenTreeStmt. (dotnet/coreclr#26668)

* Rename `GenTreeStmt` to `Statement`.

* Move `Statements::IsPhiDefnStmt` implementation to the header.

* Add iterators for statements.

So we have less places where we access Statement's fields.

* Add a header for `fgInsertStmtListAfter`,

# Conflicts:
#	src/jit/flowgraph.cpp

* Change `fgInsertStmtAtBeg` not to return a value.

and add headers for `fgInsertStmtAtBeg` and `fgNewStmtAtBeg`.

* Change `fgInsertStmtBefore` and `fgInsertStmtAfter` not to return.

* Change `fgInsertStmtNearEnd` not to return.

* Change `fgInsertStmtAtEnd` not to return.

# Conflicts:
#	src/jit/flowgraph.cpp

* Fix comments.

* Add a header for `gtDispBlockStmts`.

Commit migrated from https://github.com/dotnet/coreclr/commit/ef585a4658a830945f24d635b234f44bd0c81e84