GVN: merge overflow intrinsics with non-overflow instructions.

When an overflow intrinsic is followed by a non-overflow instruction,
replace the latter with an extract. For example:

  %sadd = tail call { i32, i1 } @llvm.sadd.with.overflow.i32(i32 %a, i32 %b)
  %sadd3 = add i32 %a, %b

Here the add statement will be replaced by an extract.

When an overflow intrinsic follows a non-overflow instruction, a clone
of the intrinsic is inserted before the normal instruction, which makes
it the same as the previous case. Subsequent runs of GVN can then clean
up the duplicate instructions and insert the extract.

This fixes PR8817.