cmd/compile: change irgen to generate exprs/stmts after decls processed

This CL changes irgen to wait until all top-level declarations have
been processed before constructing any expressions or statements that
reference them. This is the same approach that typecheck used.

Mechanically, it splits varDecl and funcDecl (the two top-level
declarations that can generate/contain code) into a part that runs
immediately for constructing the ir.ONAME, and then a separate task
that runs later to handle the code.

It also adds an exprStmtOK flag to indicate when it's actually safe to
start constructing (non-trivial) expressions and statements.

Fixes #47928.

Change-Id: I51942af6823aa561d341e2ffc1142948da025fa2
Reviewed-on: https://go-review.googlesource.com/c/go/+/344649
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dan Scales <danscales@google.com>

src/cmd/compile/internal/noder/expr.go

@@ -50,6 +50,8 @@ func (g *irgen) expr(expr syntax.Expr) ir.Node {
 		base.FatalfAt(g.pos(expr), "unrecognized type-checker result")
 	}
 
+	base.Assert(g.exprStmtOK)
+
 	// The gc backend expects all expressions to have a concrete type, and
 	// types2 mostly satisfies this expectation already. But there are a few
 	// cases where the Go spec doesn't require converting to concrete type,