`-passes='default<O3>'` isn't correctly parsed on DOS, so when update_test_checks.py runs a system call on the opt RUN line, it fails to evaluate properly - use `-passes="default<O3>"` instead.

If the gep is nusw (usually via inbounds) and the offset is
non-negative, we can infer nuw.

Proof: https://alive2.llvm.org/ce/z/ihztLy

We are replacing a narrow IV increment with a wider one. If the original
(narrow) increment did not wrap, the wider one should not wrap either.
Set the flags to be the union of both wide increment and original
increment; this ensures we preserve flags SCEV could infer for the wider
increment.

Fixes https://github.com/llvm/llvm-project/issues/71517.

Reverts llvm/llvm-project#71054

ReassociatePass may clear nsw/nuw flags of some instructions, which may
have side effects on optimizations in IndVarSimplifyPass.

Builds on #67982 which recently introduced the nneg flag on a zext
instruction.

Required manually running update_test_checks:
  AArch64/hoisting-sinking-required-for-vectorization.ll
  AArch64/peel-multiple-unreachable-exits-for-vectorization.ll
  ARM/arm_mult_q15.ll
  X86/hoist-load-of-baseptr.ll
  X86/spurious-peeling.ll

This reverts commit e91fe08999d5f5d7e7777837c529bac692d06c1b.

Breaks following buildbots: https://lab.llvm.org/buildbot/#/builders/171

In D110057 we moved LoopFlatten to a LoopPassManager. This caused a performance
regression for our 64-bit targets (the 32-bit were unaffected), the pass is no
longer triggering for a motivating example. The reason is that the IR is just
very different than expected; we try to match loop statements and particular
uses of induction variables. The easiest is to just move LoopFlatten to a place
in the pipeline where the IR is as expected, which is just before
IndVarSimplify. This means we move it from LPM2 to LPM1, so that it actually
runs just a bit earlier from where it was running before. IndVarSimplify is
responsible for significant rewrites that are difficult to "look through" in
LoopFlatten.

Differential Revision: https://reviews.llvm.org/D116612