Fix comparsion of {CLOBBER} in icf

after fixing few issues I gotto stage where 1.4M icf mismatches are due to
comparing two gimple clobber.  The problem is that operand_equal_p match
clobber

case CONSTRUCTOR:
 /* In GIMPLE empty constructors are allowed in initializers of
    aggregates.  */
 return !CONSTRUCTOR_NELTS (arg0) && !CONSTRUCTOR_NELTS (arg1);

But this happens too late after comparing its types (that are not very relevant
for memory store).

In the context of ipa-icf we do not really need to match RHS of gimple clobbers:
it is enough to know that the LHS stores can be considered equivalent.

I this added logic to hash them all the same way and compare using
TREE_CLOBBER_P flag.  I see other option in extending operand_equal_p
in fold-const to handle them more generously or making stmt hash and compare
to skip comparing/hashing RHS of gimple_clobber_p.

	* ipa-icf-gimple.c (func_checker::hash_operand): Hash gimple clobber.
	(func_checker::operand_equal_p): Special case gimple clobber.