[InstCombine] convert lshr to ashr to eliminate cast op

This is similar to b865eead ( D103617 ) and fixes:
https://llvm.org/PR50575

41b71f71 did this and more (noted with TODO
comments in the tests), but it didn't handle the case
where the destination is narrower than the source, so
it got reverted.

This is a simple match-and-replace. If there's evidence
that the TODO cases are useful, we can revisit/extend.