[SAO] Add RedundantMoveValueElimination.
Adds to SemanticARCOpts a new step of removing move_value instructions
if they are redundant. A move_value is redundant if it adds no new
information or optimization opportunities.
An example of adding information: a lifetime becoming lexical. The new
lifetime's destroys cannot be hoisted over deinit barriers.
An example of adding an optimization opportunity: the original value
escapes but the value produced by the move_value does not escape. So
destroys of the new value can be hoisted more aggressively.