ipa/105676 - pure attribute suggestion for const function
When a function is declared const (even though it technically
accesses memory), ipa-modref discovering pureness shouldn't end
up suggesting that attribute. The following thus exempts
'const' functions from ipa_make_function_pure handling.
PR ipa/105676
* ipa-pure-const.cc (ipa_make_function_pure): Skip also
for functions already being const.
* gcc.dg/pr105676.c: New testcase.
(cherry picked from commit
45e09c2eb9c2bdd34ef777e06ddc9908dd0664f9)