Introduce vNULL to use as a nil initializer for vec<>.

This patch implements Jakub's idea of adding an empty struct with
a typecast operator that returns nil vectors.  This is useful to
shorten all the initializers when declaring vec instances or passing
nil vectors as function arguments.

2012-11-20  Diego Novillo  <dnovillo@google.com>
	    Jakub Jelinek  <jakub@redhat.com>

ChangeLog
	* vec.h (struct vnull): Declare.
	(vNULL): Declare.
	* vec.c (vNULL): Define.
	* bb-reorder.c: Replace all vec<T, A>() initializers with vNULL.
	* cfgexpand.c: Likewise.
	* cfgloop.c: Likewise.
	* cfgloopanal.c: Likewise.
	* cfgloopmanip.c: Likewise.
	* cgraph.c: Likewise.
	* config/c6x/c6x.c: Likewise.
	* config/i386/i386.c: Likewise.
	* df-core.c: Likewise.
	* dominance.c: Likewise.
	* dwarf2out.c: Likewise.
	* except.c: Likewise.
	* function.c: Likewise.
	* gcse.c: Likewise.
	* genautomata.c: Likewise.
	* graphds.c: Likewise.
	* graphite-scop-detection.c: Likewise.
	* graphite.c: Likewise.
	* haifa-sched.c: Likewise.
	* ifcvt.c: Likewise.
	* ipa-cp.c: Likewise.
	* ipa-inline-analysis.c: Likewise.
	* ipa-inline-transform.c: Likewise.
	* ipa-inline.c: Likewise.
	* ipa-prop.c: Likewise.
	* ipa-split.c: Likewise.
	* ipa-utils.c: Likewise.
	* ira-build.c: Likewise.
	* lto-cgraph.c: Likewise.
	* lto-streamer-in.c: Likewise.
	* lto-symtab.c: Likewise.
	* opts-global.c: Likewise.
	* passes.c: Likewise.
	* ree.c: Likewise.
	* sched-deps.c: Likewise.
	* sel-sched-ir.c: Likewise.
	* sel-sched-ir.h: Likewise.
	* sel-sched.c: Likewise.
	* stor-layout.c: Likewise.
	* trans-mem.c: Likewise.
	* tree-call-cdce.c: Likewise.
	* tree-cfg.c: Likewise.
	* tree-data-ref.c: Likewise.
	* tree-diagnostic.c: Likewise.
	* tree-eh.c: Likewise.
	* tree-loop-distribution.c: Likewise.
	* tree-predcom.c: Likewise.
	* tree-ssa-loop-im.c: Likewise.
	* tree-ssa-loop-ivcanon.c: Likewise.
	* tree-ssa-loop-manip.c: Likewise.
	* tree-ssa-loop-niter.c: Likewise.
	* tree-ssa-loop-prefetch.c: Likewise.
	* tree-ssa-math-opts.c: Likewise.
	* tree-ssa-phiopt.c: Likewise.
	* tree-ssa-pre.c: Likewise.
	* tree-ssa-propagate.c: Likewise.
	* tree-ssa-reassoc.c: Likewise.
	* tree-ssa-sccvn.c: Likewise.
	* tree-ssa-structalias.c: Likewise.
	* tree-ssa-threadedge.c: Likewise.
	* tree-ssa-uninit.c: Likewise.
	* tree-stdarg.c: Likewise.
	* tree-switch-conversion.c: Likewise.
	* tree-vect-data-refs.c: Likewise.
	* tree-vect-loop.c: Likewise.
	* tree-vect-slp.c: Likewise.
	* tree-vect-stmts.c: Likewise.
	* value-prof.c: Likewise.
	* varasm.c: Likewise.

ada/ChangeLog
	* gcc-interface/decl.c: Replace all vec<T,A>()
	initializers with vNULL.

cp/ChangeLog
	* name-lookup.c: Replace all vec<T, A>() initializers
	with vNULL.
	* semantics.c: Likewise.

fortran/ChangeLog
	* trans-openmp.c: Replace all vec<T, A>() initializers
	with vNULL.

lto/ChangeLog
	* lto.c: Replace all vec<T, A>() initializers with vNULL.

objc/ChangeLog
	* objc-act.c: Replace all vec<T, A>() initializers with vNULL.

Co-Authored-By: Jakub Jelinek <jakub@redhat.com>

From-SVN: r193677