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.

gcc/testsuite/gcc.dg/vect/pr98302.c

@@ -0,0 +1,22 @@
+#include "tree-vect.h"
+
+int c = 1705;
+char a;
+long f = 50887638;
+unsigned long long *h(unsigned long long *k, unsigned long long *l) {
+  return *k ? k : l;
+}
+void aa() {}
+int main() {
+  check_vect ();
+
+  long d = f;
+  for (char g = 0; g < (char)c - 10; g += 2) {
+    unsigned long long i = d, j = 4;
+    a = *h(&i, &j) << ((d ? 169392992 : 0) - 169392955LL);
+  }
+  if (a)
+    __builtin_abort();
+
+  return 0;
+}

gcc/tree-vect-patterns.c

@@ -4961,10 +4961,19 @@ vect_determine_precisions_from_users (stmt_vec_info stmt_info, gassign *stmt)
 	unsigned int const_shift = TREE_INT_CST_LOW (shift);
 	if (code == LSHIFT_EXPR)
 	  {
+	    /* Avoid creating an undefined shift.
+
+	       ??? We could instead use min_output_precision as-is and
+	       optimize out-of-range shifts to zero.  However, only
+	       degenerate testcases shift away all their useful input data,
+	       and it isn't natural to drop input operations in the middle
+	       of vectorization.  This sort of thing should really be
+	       handled before vectorization.  */
+	    operation_precision = MAX (stmt_info->min_output_precision,
+				       const_shift + 1);
 	    /* We need CONST_SHIFT fewer bits of the input.  */
-	    operation_precision = stmt_info->min_output_precision;
 	    min_input_precision = (MAX (operation_precision, const_shift)
-				    - const_shift);
+				   - const_shift);
 	  }
 	else
 	  {