Fix LSR compile time.

This is a simple fix that brings the compilation time from 5min to 5s
on a specific real-world example. It's a large chain of computation in
a crypto routine (always a problem for SCEV). A unit test is not
feasible and there would be no way to check it. The fix is just basic
good practice for dealing with SCEVs, there's no risk of regression.

Patch by Daniel Reynaud!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220622 91177308-0d34-0410-b5e6-96231b3b80d8
LSR: Minor cleanup after Daniel's patch.

Combine the Inserted an Done sets into a Visited set.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220623 91177308-0d34-0410-b5e6-96231b3b80d8