SPV: Flatten structs for copy when they are GLSL type aliases.

Addresses issue #304 and issue #307 by replacing unmatched type OpStores with
per-member copies. Covers assignment statements and most argument passing, but
does not yet cover r-value-based argument passing.

Test/spv.multiStructFuncall.frag

@@ -0,0 +1,21 @@
+#version 450
+
+struct S { mat4 m; };
+buffer blockName { S s1; };  // need an S with decoration
+S s2;                        // no decorations on S
+
+void fooConst(const in S s) { }
+void foo(in S s) { }
+void fooOut(inout S s) { }
+
+void main()
+{
+  fooConst(s1);
+  fooConst(s2);
+
+  foo(s1);
+  foo(s2);
+
+  fooOut(s1);
+  fooOut(s2);
+}