Improve hashing of decls in ipa-icf-gimple

Another remaining case is that we end up comparing calls with mismatching
number of parameters or with different permutations of them.

This is because we hash decls to nothing. This patch improves that by
hashing decls by their code and parm decls by indexes that are stable.
Also for defualt defs in SSA_NAMEs we can add the corresponding decl (that
is usually parm decls).

Still we could improve on this by hasing ssa names by their definit parameters
and possibly making maps of other decls and assigning them stable function
local IDs.

	* ipa-icf-gimple.c (func_checker::hash_operand): Improve hashing of
	decls.