[Bug tree-optimization/107493] [13 Regression] Wrong code at -Os on x86_64-linux-gnu

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107493

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
a_lsm.9_12 = PHI <-21(2), _4(3)>

(set_scalar_evolution
  instantiated_below = 2
  (scalar = a_lsm.9_12)
  (scalar_evolution = (int) {65515, +, 10}_1))

is definitely wrong since the initial value is off.  The evolution after
that looks OK though, so not sure why SCCP gets the wrong final value here,
we compute the number of iterations to zero (also wrong).

It goes wrong when we get to the truncation, we then do

        *evolution_of_loop = chrec_convert (type, *evolution_of_loop, at_stmt);

with {-21, + 2}_1 converting that to unsigned short.  This ends up in
convert_affine_scev but that assumes it converts the cycle and not that
the conversion is inside the cycle.

The old sanity checking of the initial value would have catched this but the
actual error is elsewhere.  The question is what's valid here.  I'm going to
try restricting this all further.