Added BASIC-SAVE-TN which must be used in lieu of SAVE-COMPILEX-WRITER-TN
in spilling, since the TN might already be a saved single-writer TN. Fixed OPTIMIZED-EMIT-SAVES to choose :SAVE-ONCE saving strategy when appropriate. This had come unglued, since there was always a restore emitted by the time we considered saving, making it look like the TN wasn't single-writer. Changed SELECT-LOCATIONS to not choose SC-RESERVE-LOCATIONS. Changed load-TN packing to handle multi-location load-tns. Changed load-TN packing to allow targeting into TNs that are not allowed by the primitive type.