(Last?) Semi-regular merge from tip to dev.ssa.

Conflicts:
	src/cmd/compile/internal/gc/closure.go
	src/cmd/compile/internal/gc/gsubr.go
	src/cmd/compile/internal/gc/lex.go
	src/cmd/compile/internal/gc/pgen.go
	src/cmd/compile/internal/gc/syntax.go
	src/cmd/compile/internal/gc/walk.go
	src/cmd/internal/obj/pass.go

Change-Id: Ib5ea8bf74d420f4902a9c6208761be9f22371ae7