Fix PR61657 (undefined behavior in loop-iv.c)

Fix PR61657 (undefined behavior in loop-iv.c)

bootstrap-ubsan shows:
 loop-iv.c:2626:14: runtime error: signed integer overflow: 9223372036854775806 - -9223372036854775808 cannot be represented in type 'long int'

Fixed by moving the variables in question from signed to unsigned.

	PR rtl-optimization/61657
	* loop-iv.c (iv_number_of_iterations): Declare up and down as
	unsigned. Remove superflous uint64_t cast.

From-SVN: r227075