vect: Avoid generating out-of-range shifts [PR98302]

vect: Avoid generating out-of-range shifts [PR98302]

In this testcase we end up with:

  unsigned long long x = ...;
  char y = (char) (x << 37);

The overwidening pattern realised that only the low 8 bits
of x << 37 are needed, but then tried to turn that into:

  unsigned long long x = ...;
  char y = (char) x << 37;

which gives an out-of-range shift.  In this case y can simply
be replaced by zero, but as the comment in the patch says,
it's kind-of awkward to do that in the middle of vectorisation.

Most of the overwidening stuff is about keeping operations
as narrow as possible, which is important for vectorisation
but could be counter-productive for scalars (especially on
RISC targets).  In contrast, optimising y to zero in the above
feels like an independent optimisation that would benefit scalar
code and that should happen before vectorisation.

gcc/
	PR tree-optimization/98302
	* tree-vect-patterns.c (vect_determine_precisions_from_users): Make
	sure that the precision remains greater than the shift count.

gcc/testsuite/
	PR tree-optimization/98302
	* gcc.dg/vect/pr98302.c: New test.