Allow uncontained GT_LONG nodes

Allow uncontained GT_LONG nodes

When the result of a TYP_LONG operation is not used the GT_LONG node inserted during decomposition remains in the tree and unlike other GT_LONG nodes is not contained and needs special handling.

This usually happens with arithmetic operations that require overflow checks,  the result of the operation may be ignored but the operation itself cannot be eliminated because it has a side-effect.

Commit migrated from https://github.com/dotnet/coreclr/commit/6f8f2154825b1cf17fbbba8ccb365069e2d7d15f