Changes

[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

[InstCombine] conditional sign-extend of high-bit-extract: '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
It is possible that `haveNoCommonBitsSet()` should be taught about this 
pattern so that we never have an `add` variant, but the reasoning would 
need to be recursive (because of that `select`), so i'm not really sure 
that would be worth it just yet.
llvm-svn: 375378