middle-end/100672 - fix bogus right shift folding

This fixes the bogus use of TYPE_PRECISION on vector types
from optimizing -((int)x >> 31) into (unsigned)x >> 31.

2021-05-19  Richard Biener  <rguenther@suse.de>

	PR middle-end/100672
	* fold-const.c (fold_negate_expr_1): Use element_precision.
	(negate_expr_p): Likewise.

	* gcc.dg/torture/pr100672.c: New testcase.