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

[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