[NFC][InstCombine] conditional sign-extend of high-bit-extract: 'and' pat. can be 'or' pattern.

In this pattern, all the "magic" bits that we'd add are all
high sign bits, and in the value we'd be adding to they are all unset,
not unexpectedly, so we can have an `or` there:
https://rise4fun.com/Alive/ups

llvm-svn: 375377