Revert "Revert r253253 and r253126: "Don't recompute LCSSA after loop-unrolling when possible.""

The bug in IndVarSimplify was fixed in r254976, r254977, so I'm
reapplying the original patch for avoiding redundant LCSSA recomputation.

This reverts commit ffe3b434e505e403146aff00be0c177bb6d13466.

llvm-svn: 255133