Fix crash with constant initializer caused by IPA

Fix crash with constant initializer caused by IPA

The testcase compiled with -O2 -gnatn makes the compiler crash in
vect_can_force_dr_alignment_p during SLP vectorization:

  if (decl_in_symtab_p (decl)
      && !symtab_node::get (decl)->can_increase_alignment_p ())
    return false;

because symtab_node::get (decl) returns a null node.  The phenomenon occurs
for a pair of twin symbols listed like so in .cgraph:

Opt7_Pkg.T12b/17 (Opt7_Pkg.T12b)
  Type: variable definition analyzed
  Visibility: semantic_interposition external public artificial
  Aux: @0x44d45e0
  References:
  Referring: opt7_pkg__enum_name_table/13 (addr) opt7_pkg__enum_name_table/13
(addr)
  Availability: not-ready
  Varpool flags: initialized read-only const-value-known

Opt7_Pkg.T8b/16 (Opt7_Pkg.T8b)
  Type: variable definition analyzed
  Visibility: semantic_interposition external public artificial
  Aux: @0x7f9fda3fff00
  References:
  Referring: opt7_pkg__enum_name_table/13 (addr) opt7_pkg__enum_name_table/13
(addr)
  Availability: not-ready
  Varpool flags: initialized read-only const-value-known

with:

opt7_pkg__enum_name_table/13 (Opt7_Pkg.Enum_Name_Table)
  Type: variable definition analyzed
  Visibility: semantic_interposition external public
  Aux: @0x44d45e0
  References: Opt7_Pkg.T8b/16 (addr) Opt7_Pkg.T8b/16 (addr) Opt7_Pkg.T12b/17
(addr) Opt7_Pkg.T12b/17 (addr)
  Referring: opt7_pkg__image/2 (read) opt7_pkg__image/2 (read)
opt7_pkg__image/2 (read) opt7_pkg__image/2 (read) opt7_pkg__image/2 (read)
opt7_pkg__image/2 (read) opt7_pkg__image/2 (read) opt7_pkg__image/2 (read)
  Availability: not-ready
  Varpool flags: initialized read-only const-value-known

being the crux of the matter.

What happens is that symtab_remove_unreachable_nodes leaves the last symbol
in kind of a limbo state: in .remove_symbols, we have:

opt7_pkg__enum_name_table/13 (Opt7_Pkg.Enum_Name_Table)
  Type: variable
  Body removed by symtab_remove_unreachable_nodes
  Visibility: externally_visible semantic_interposition external public
  References:
  Referring: opt7_pkg__image/2 (read) opt7_pkg__image/2 (read)
  Availability: not_available
  Varpool flags: initialized read-only const-value-known

This means that the "body" (DECL_INITIAL) of the symbol has been disregarded
during reachability analysis, causing the first two symbols to be discarded:

Reclaiming variables: Opt7_Pkg.T12b/17 Opt7_Pkg.T8b/16

but the DECL_INITIAL is explicitly preserved for later constant folding,
which makes it possible to retrofit the DECLs corresponding to the first
two symbols in the GIMPLE IR and ultimately leads to the crash.

gcc/
	* tree-vect-data-refs.cc (vect_can_force_dr_alignment_p): Return
	false if the variable has no symtab node.

gcc/testsuite/
	* gnat.dg/specs/opt7.ads: New test.
	* gnat.dg/specs/opt7_pkg.ads: New helper.
	* gnat.dg/specs/opt7_pkg.adb: Likewise.