ia32: Optimize entity creation for floatingpoint constants.

When creating an entity for a floatingpoint constant, first try to reduce the size, then check, whether it already exists.
This way only a single entity is created for e.g. a double and a float 1.0.
ia32: Replace repeated code in ia32_create_float_const_entity() by a loop.
dump: Show the debug info of constlike nodes even when dumping them node local.