[mid-end] [obvious] Cast to avoid -Wsign-compare warning from r279124

[mid-end] [obvious] Cast to avoid -Wsign-compare warning from r279124

The previous revision adds a comparison that triggers a warning.
Here we simply cast the unsigned integer to a signed one before the
comparison.

gcc/ChangeLog:

2019-12-09  Matthew Malcomson  <matthew.malcomson@arm.com>

	* regstat.c (regstat_bb_compute_calls_crossed): Add a cast to
	avoid a warning.

From-SVN: r279126