[NFC][InstCombine] Shift amount reassociation: add flag preservation test

As discussed in https://reviews.llvm.org/D63812#inline-569870
* exact on both lshr => exact https://rise4fun.com/Alive/plHk
* exact on both ashr => exact https://rise4fun.com/Alive/QDAA
* nuw on both shl => nuw https://rise4fun.com/Alive/5Uk
* nsw on both shl => nsw https://rise4fun.com/Alive/0plg

So basically if the same flag is set on both original shifts -> set it on new shift.
Don't think we can do anything with non-matching flags on shl.

llvm-svn: 364652
1 file changed
tree: 14d4fb76e3e7b47dfc1ac922fcea988ac0723851
  1. clang/
  2. clang-tools-extra/
  3. compiler-rt/
  4. debuginfo-tests/
  5. libclc/
  6. libcxx/
  7. libcxxabi/
  8. libunwind/
  9. lld/
  10. lldb/
  11. llgo/
  12. llvm/
  13. openmp/
  14. parallel-libs/
  15. polly/
  16. pstl/
  17. .arcconfig
  18. .clang-format
  19. .clang-tidy
  20. .gitignore
  21. README.md
README.md

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