middle-end/100547 - check rtvec_alloc size

This makes the rtvec_alloc argument size_t catching overflow and
truncated arguments (from "invalid" testcases), verifying the
argument against INT_MAX which is the limit set by the int
typed rtvec_def.num_elem member.

2021-05-12  Richard Biener  <rguenther@suse.de>

	PR middle-end/100547
	* rtl.h (rtvec_alloc): Make argument size_t.
	* rtl.c (rtvec_alloc): Verify the count is less than INT_MAX.