draw: Ensure that the vertex_header type size matches expectation.

This is failing sometimes, probably because TargetData keeps a structure layout
cache, which can becomes bogus, ever since the InvalidateStructLayoutInfo API
was removed in LLVM r135245.

This change merely makes the problem easier to diagnose (an assertion
failure instead of a random crash).

src/gallium/auxiliary/draw/draw_llvm.c

@@ -305,6 +305,9 @@ create_jit_vertex_header(struct gallivm_state *gallivm, int data_elems)
                           target, vertex_header,
                           DRAW_JIT_VERTEX_DATA);
 
+   assert(LLVMABISizeOfType(target, vertex_header) ==
+          offsetof(struct vertex_header, data[data_elems]));
+
    return vertex_header;
 }