Mark GT_IND nodes that represent VSD targets.

Mark GT_IND nodes that represent VSD targets.

The operand to such a node must be materialized into a register on
certain platforms. This change defines a new IND-specific flag,
GTF_IND_VSD_TGT, and uses that flag to indicate that a particular
GT_IND node represents a VSD target. This flag is then observed by
lowering on the necessary platforms in order to short circuit the
logic that attempts to make the GT_IND's operand contained.

Commit migrated from https://github.com/dotnet/coreclr/commit/0188c07e524986bd6f8587e57f37f97e3143bb0e