Refactor isContained().

Refactor isContained().

This changes isContained to terminate early if it is not possible for the node in question
to ever be contained. There are many nodes that are statically non-containable: any node
that is not a value may not be contained along with a small number of value nodes. To avoid
the need for a `switch` to identify the latter, their node kinds have been marked with a
new flag, `GTK_NOCONTAIN`. The checks for identifying non-containable nodes are encapsulated
withing a new function, `canBeContained`.

Commit migrated from https://github.com/dotnet/coreclr/commit/350020d9b6a2bda08cf2e416d959c1801a717734