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

The change exposed a bug in IndVarSimplify (PR25578), which led to a
failure (PR25538). When the bug is fixed, this patch can be reapplied.

The tests are kept in tree, as they're useful anyway, and will not break
with this revert.

llvm-svn: 253596