[llvm] [VPlan] Extract reverse mask from reverse accesses (PR #155579)

[Luke Lau via llvm-commits]

lukel97 wrote:

I think one of the aarch64 buildbots is failing on llvm-test-suite with this: https://lab.llvm.org/buildbot/#/builders/198/builds/12563

```
clang: /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/llvm/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp:3840: virtual InstructionCost llvm::VPWidenMemoryRecipe::computeCost(ElementCount, VPCostContext &) const: Assertion `!IsReverse() && "Inconsecutive memory access should not have reverse order"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.      Program arguments: /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/stage1.install/bin/clang -DNDEBUG -mcpu=neoverse-v2 -mllvm -scalable-vectorization=preferred -O3 -std=gnu17 -fcommon -Wno-error=incompatible-pointer-types -MD -MT MultiSource/Benchmarks/Prolangs-C/TimberWolfMC/CMakeFiles/timberwolfmc.dir/finalpin.c.o -MF CMakeFiles/timberwolfmc.dir/finalpin.c.o.d -o CMakeFiles/timberwolfmc.dir/finalpin.c.o -c /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/test/test-suite/MultiSource/Benchmarks/Prolangs-C/TimberWolfMC/finalpin.c
```

https://github.com/llvm/llvm-project/pull/155579