Fix PR tree-optimization/78598 - tree-ssa-loop-prefetch.c:835:16: runtime error:...

Fix PR tree-optimization/78598 - tree-ssa-loop-prefetch.c:835:16: runtime error: signed integer overflow

Using bootstrap-ubsan gcc to build mplayer shows:

tree-ssa-loop-prefetch.c:835:16: runtime error: signed integer overflow:
288230376151711743 * 64 cannot be represented in type 'long int'

Here signed und unsigned integers are mixed in a division resulting in
bogus values: (-83 + 64ULL -1) / 64ULL) == 288230376151711743

Fixed by casting the unsigned parameter to signed.

	PR tree-optimization/78598
	* tree-ssa-loop-prefetch.c (ddown): Cast to signed to avoid
	overflows.

From-SVN: r243113