[TailCallElim] Drop poison-generating flags on reassociated accumulators (#200624)

For example if you have recursion like

    int prod(n) {
      if (n == 0) return 1;
      return prod(n-1) * f(n)
    }

then logically this computes (((f(1) * f(2)) * f(3)) * f(4)) * ... f(n).

But TailCallElim reassociates this, computing instead
((f(n) * f(n-1)) * f(n-2)) * ...

If the operator (* in this case) had poison-generating flags like
nsw, those may not still apply after reassociation.  (For example,
suppose in this example f(1) returns 0 -- in that case the original
multiplication cannot overflow, but the new one still might.)

Fix this by clearing the poison-generating flags after reassociating.