[InstCombine] erase instructions leading up to unreachable

Normal dead code elimination ignores assume intrinsics, so we fail to
delete assumes that are not meaningful (and potentially worse if they
cause conflicts with other assumptions).

The motivating example in https://llvm.org/PR47416 suggests that we
might have problems upstream from here (difference between C and C++),
but this should be a cheap way to make sure we remove more dead code.

Differential Revision: https://reviews.llvm.org/D87149