tree-cfg: Improve gimple switch verification

tree-cfg: Improve gimple switch verification

When looking at the verification, I have noticed a bug in it.

The verification that CASE_HIGH (if present) has the same type as CASE_LOW
is only performed for the case label 2 and higher, case label 1 (the first
one after the default label) isn't checked.

The following patch fixes that, it will uselessly also compare
TREE_TYPE (CASE_LOW (elt)) != elt_type for the case label 1, but I think
that isn't that expensive and helps readability of the code.

2020-08-31  Jakub Jelinek  <jakub@redhat.com>

	* tree-cfg.c (verify_gimple_switch): If the first non-default case
	label has CASE_HIGH, verify it has the same type as CASE_LOW.