compiler: record pointer var values to remove write barriers

compiler: record pointer var values to remove write barriers

    Record when a local pointer variable is set to a value such that
    indirecting through the pointer does not require a write barrier.  Use
    that to eliminate write barriers when indirecting through that local
    pointer variable.  Only keep this information per-block, so it's not
    all that applicable.

    This reduces the number of write barriers generated when compiling the
    runtime package from 553 to 524.

    The point of this is to eliminate a bad write barrier in the bytes
    function in runtime/print.go.  Mark that function nowritebarrier so
    that the problem does not recur.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/191581

From-SVN: r274890