[DivRemPairs] Don't assert that we won't ever get expanded-form rem pairs in...

[DivRemPairs] Don't assert that we won't ever get expanded-form rem pairs in different BB's (PR43500)

If we happen to have the same div in two basic blocks,
and in one of those we also happen to have the rem part,
we'd match the div-rem pair, but the wrong ones.
So let's drop overly-ambiguous assert.

Fixes https://bugs.llvm.org/show_bug.cgi?id=43500

llvm-svn: 373167