gimplify: Don't optimize register const vars to static [PR93949]

gimplify: Don't optimize register const vars to static [PR93949]

The following testcase is rejected, while it was accepted in 3.4 and earlier
(before tree-ssa merge).
The problem is that we decide to promote the const variable to TREE_STATIC,
but TREE_STATIC DECL_REGISTER VAR_DECLs may only be the global register vars
and so assemble_variable/make_decl_rtl diagnoses it.

Either we do what the following patch does, where we could consider
register as a hint the user doesn't want such optimization, because if
something is forced static, it is not "register" anymore and register static
is not valid in C either, or we could clear DECL_REGISTER instead, but would
still need to punt at least on DECL_HARD_REGISTER cases.

2020-02-27  Jakub Jelinek  <jakub@redhat.com>

	PR c/93949
	* gimplify.c (gimplify_init_constructor): Don't promote readonly
	DECL_REGISTER variables to TREE_STATIC.

	* gcc.c-torture/compile/pr93949.c: New test.