ira: Recompute regstat as max_regno changes [PR97705]

As PR97705 shows, the commit r11-4637 caused some dumping
comparison difference error on pass ira.  It exposed one
issue about the newly introduced function remove_scratches,
which can increase the largest pseudo reg number if it
succeeds, later some function will use the max_reg_num()
to get the latest max_regno, when iterating the numbers
we can access some data structures which are allocated as
the previous max_regno, some out of array bound accesses
can occur, the failure can be random since the values
beyond the array could be random.

This patch is to free/reinit/recompute the relevant data
structures that is regstat_n_sets_and_refs and reg_info_p
to ensure we won't access beyond some array bounds.

Bootstrapped/regtested on powerpc64le-linux-gnu P9 and
powerpc64-linux-gnu P8.

gcc/ChangeLog:

	PR rtl-optimization/97705
	* ira.c (ira): Refactor some regstat free/init/compute invocation
	into lambda function regstat_recompute_for_max_regno, and call it
	when max_regno increases as remove_scratches succeeds.