[LoopDataPrefetch][AArch64FalkorHWPFFix] Preserve ScalarEvolution

[LoopDataPrefetch][AArch64FalkorHWPFFix] Preserve ScalarEvolution

Summary:
Mark LoopDataPrefetch and AArch64FalkorHWPFFix passes as preserving
ScalarEvolution since they do not alter loop structure and should not
alter any SCEV values (though LoopDataPrefetch may introduce new
instructions that won't have cached SCEV values yet).

This can result in slight code differences, mainly w.r.t. nsw/nuw flags
on SCEVs, since these are computed somewhat lazily when a zext/sext
instruction is encountered.  As a result, passes after the modified
passes may see SCEVs with more nsw/nuw flags present.

Reviewers: sanjoy, anemet

Subscribers: aemerson, rengolin, mzolotukhin, javed.absar, kristof.beyls, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D36716

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@311032 91177308-0d34-0410-b5e6-96231b3b80d8