tree-optimization/61247 - handle peeled converted IV in SCEVHEADtrunkmaster

The following handles SCEV analysis of a peeled converted IV if
that IV is known to not overflow.  For

  # _15 = PHI <_4(6), 0(5)>
  # i_18 = PHI <i_11(6), 0(5)>
  i_11 = i_18 + 1;
  _4 = (long unsigned int) i_11;

we cannot analyze _15 directly since the SCC has a widening
conversion.  But we can analyze _4 to (long unsigned int) {1, +, 1}_1
which is "peeled" (it's from after the first iteration of _15).
If the un-peeled IV {0, +, 1}_1 has the same initial value as _15
and it does not overflow then _15 can be analyzed as
{0ul, +, 1ul}_1.

The following implements this in simplify_peeled_chrec.

	PR tree-optimization/61247
	* tree-scalar-evolution.cc (simplify_peeled_chrec):
	Handle the case of a converted peeled chrec.

	* gcc.dg/vect/vect-pr61247.c: New testcase.

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