[InstCombine] Bypass high bit extract before variable sign-extension (PR43523)

https://rise4fun.com/Alive/8BY - valid for lshr+trunc+variable sext
https://rise4fun.com/Alive/7jk - the variable sext can be redundant

https://rise4fun.com/Alive/Qslu - 'exact'-ness of first shift can be preserver

https://rise4fun.com/Alive/IF63 - without trunc we could view this as
                                  more general "drop redundant mask before right-shift",
                                  but let's handle it here for now
https://rise4fun.com/Alive/iip - likewise, without trunc, variable sext can be redundant.

There's more patterns for sure - e.g. we can have 'lshr' as the final shift,
but that might be best handled by some more generic transform, e.g.
"drop redundant masking before right-shift" (PR42456)

I'm singling-out this sext patch because you can only extract
high bits with `*shr` (unlike abstract bit masking),
and i *know* this fold is wanted by existing code.

I don't believe there is much to review here,
so i'm gonna opt into post-review mode here.

https://bugs.llvm.org/show_bug.cgi?id=43523

llvm-svn: 373542