value-range: Give up on POLY_INT_CST ranges [PR97457]

value-range: Give up on POLY_INT_CST ranges [PR97457]

This PR shows another problem with calculating value ranges for
POLY_INT_CSTs.  We have:

  ivtmp_76 = ASSERT_EXPR <ivtmp_60, ivtmp_60 > POLY_INT_CST [9, 4294967294]>

where the VQ coefficient is unsigned but is effectively acting
as a negative number.  We wrongly give the POLY_INT_CST the range:

  [9, INT_MAX]

and things go downhill from there: later iterations of the unrolled
epilogue are wrongly removed as dead.

I guess this is the final nail in the coffin for doing VRP on
POLY_INT_CSTs.  For other similarly exotic testcases we could have
overflow for any coefficient, not just those that could be treated
as contextually negative.

Testing TYPE_OVERFLOW_UNDEFINED doesn't seem like an option because we
couldn't handle warn_strict_overflow properly.  At this stage we're
just recording a range that might or might not lead to strict-overflow
assumptions later.

It still feels like we should be able to do something here, but for
now removing the code seems safest.  It's also telling that there
are no testsuite failures on SVE from doing this.

gcc/
	PR tree-optimization/97457
	* value-range.cc (irange::set): Don't decay POLY_INT_CST ranges
	to integer ranges.

gcc/testsuite/
	PR tree-optimization/97457
	* gcc.dg/vect/pr97457.c: New test.