This is the gofrontend version of https://golang.org/cl/91796.
    
    This is part of that CL, just the compiler change and required runtime
    changes, in preparation for updating libgo to 1.11.
    
    Relevant part of original CL description:
    
        The hmap field in the maptype is only used by the runtime to check the sizes of
        the hmap structure created by the compiler and runtime agree.
    
        Comments are already present about the hmap structure definitions in the
        compiler and runtime needing to be in sync.
    
    Reviewed-on: https://go-review.googlesource.com/130976

From-SVN: r263941