Hoist spills when the same value is known to be in less loopy sibling registers.

Stack slot real estate is virtually free compared to registers, so it is
advantageous to spill earlier even though the same value is now kept in both a
register and a stack slot.

Also eliminate redundant spills by extending the stack slot live range
underneath reloaded registers.

This can trigger a dead code elimination, removing copies and even reloads that
were only feeding spills.

llvm-svn: 127868