Add an empty constructor shortcut to build_vector_from_ctor

Empty vector constructors are equivalent to zero vectors.  If we handle
that case directly, we can support it for variable-length vectors and
can hopefully make things more efficient for fixed-length vectors.

This is needed by a later C++ patch.

2019-11-18  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree.c (build_vector_from_ctor): Directly return a zero vector for
	empty constructors.

From-SVN: r278413

gcc/ChangeLog

 2019-11-18  Richard Sandiford  <richard.sandiford@arm.com>
 
+	* tree.c (build_vector_from_ctor): Directly return a zero vector for
+	empty constructors.
+
+2019-11-18  Richard Sandiford  <richard.sandiford@arm.com>
+
 	* cse.c (cse_insn): Delete no-op register moves too.
 	* simplify-rtx.c (comparison_to_mask): Handle unsigned comparisons.
 	Take a second comparison to control the value for NE.

gcc/tree.c

@@ -1858,6 +1858,9 @@ make_vector (unsigned log2_npatterns,
 tree
 build_vector_from_ctor (tree type, vec<constructor_elt, va_gc> *v)
 {
+  if (vec_safe_length (v) == 0)
+    return build_zero_cst (type);
+
   unsigned HOST_WIDE_INT idx, nelts;
   tree value;