Blank node must be ignored when building arguments substitued tree.
Otherwise, it could be used to replace other blank node in left hand
side of an assignment, causing an invalid IR node.

Consider the following code:

	type S1 struct {
		s2 S2
	}

	type S2 struct{}

	func (S2) Make() S2 {
		return S2{}
	}

	func (S1) Make() S1 {
		return S1{s2: S2{}.Make()}
	}

	var _ = S1{}.Make()

After staticAssignInlinedCall, the assignment becomes:

	var _ = S1{s2: S2{}.Make()}

and the arg substitued tree is "map[*ir.Name]ir.Node{_: S1{}}". Now,
when doing static assignment, if there is any assignment to blank node,
for example:

	_ := S2{}

That blank node will be replaced with "S1{}":

	S1{} := S2{}

So constructing an invalid IR which causes the ICE.

Fixes golang#58325

Change-Id: I21b48357f669a7e02a7eb4325246aadc31f78fb9
Reviewed-on: https://go-review.googlesource.com/c/go/+/465098
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: David Chase <drchase@google.com>