[dev.regabi] cmd/compile: cleanup for concrete types - sinit

An automated rewrite will add concrete type assertions after
a test of n.Op(), when n can be safely type-asserted
(meaning, n is not reassigned a different type, n is not reassigned
and then used outside the scope of the type assertion,
and so on).

This sequence of CLs handles the code that the automated
rewrite does not: adding specific types to function arguments,
adjusting code not to call n.Left() etc when n may have multiple
representations, and so on.

This CL focuses on sinit.go.

Passes buildall w/ toolstash -cmp.

Change-Id: I3e9458e69a7a9b3f2fe139382bf961bc4473cc42
Reviewed-on: https://go-review.googlesource.com/c/go/+/277928
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/ir/node.go

@@ -733,8 +733,6 @@ func NodAt(pos src.XPos, op Op, nleft, nright Node) Node {
 		return newNameAt(pos, op, nil)
 	case OMAKECHAN, OMAKEMAP, OMAKESLICE, OMAKESLICECOPY:
 		return NewMakeExpr(pos, op, nleft, nright)
-	case OMETHEXPR:
-		return NewMethodExpr(pos, nleft, nright)
 	case ONIL:
 		return NewNilExpr(pos)
 	case OPACK: