This is an alternate solution to rL322058 which causes performance
degradation according to report from @eastig.
Generally when we expand a SCEV and re-use the instruction we
check whether SCEV has lost a poison flags but instruction has.
In this case we just clear poison flags from instruction.
This is pretty similar what GVN will do if we we literally expand the SCEV
without poison flags.
Potentially it might have negative performance impact due to we clear
nuw/nsw from instruction and possible loose some performance
opportunities.
s/recursevely/recursively/