compiler: statically allocate constant interface data

compiler: statically allocate constant interface data

    When converting a constant to interface, such as interface{}(42)
    or interface{}("hello"), if the interface escapes, we currently
    generate a heap allocation to hold the constant value.

    This CL changes it to generate a static allocation instead, as
    the gc compiler does. This reduces allocations in such cases.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/180277

From-SVN: r271945