varpool: Restore GENERIC TREE_READONLY automatic var optimization [PR7260]

In 4.8 and earlier we used to fold the following to 0 during GENERIC folding,
but we don't do that anymore because ctor_for_folding etc. has been turned into a
GIMPLE centric API, but as the testcase shows, it is invoked even during
GENERIC folding and there the automatic vars still should have meaningful
initializers.  I've verified that the C++ FE drops TREE_READONLY on
automatic vars with const qualified types if they require non-constant
(runtime) initialization.

2021-01-27  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/97260
	* varpool.c: Include tree-pass.h.
	(ctor_for_folding): In GENERIC return DECL_INITIAL for TREE_READONLY
	non-TREE_SIDE_EFFECTS automatic variables.

	* gcc.dg/tree-ssa/pr97260.c: New test.