Detect sources produced by non-CommaOk TypeAssert of a Pointer type.

[mlevesquedion]

Consider this test case:

func TestSourcePointerAssertedFromParameterEface(e interface{}) {
	s := e.(*core.Source)
	core.Sink(s) // TODO want "a source has reached a sink"
}

The SSA is:

func TestSourcePointerAssertedFromParameterEface(e interface{})
0: entry
	0(*ssa.TypeAssert     ): t0 = typeassert e.(*example.com/core.Source)
	1(*ssa.Alloc          ): t1 = new [1]interface{} (varargs)
	2(*ssa.IndexAddr      ): t2 = &t1[0:int]
	3(*ssa.MakeInterface  ): t3 = make interface{} <- *example.com/core.Source (t0)
	4(*ssa.Store          ): *t2 = t3
	5(*ssa.Slice          ): t4 = slice t1[:]
	6(*ssa.Call           ): t5 = example.com/core.Sink(t4...)
	7(*ssa.Return         ): return

It it were a CommaOk TypeAssert, there would be an Extract, so the Source would be detected in sources.go:sourcesFromBlocks.
If it were a TypeAssert to a Source value (not a pointer), there would be an Alloc, so again the source would be detected in sources.go:sourcesFromBlocks.