Improve Upper Vector Save/Restore (#23344)

Improve Upper Vector Save/Restore (#23344)

Improve Upper Vector Save/Restore

In order to avoid saving and restore the upper half of large vectors around every
call even if they are not used, separately model the upper half of large vector
lclVars, and track whether the large vector lclVar is partially-spilled, in which
case its upper half resides in its upper half Interval's location.

Fix #18144