compiler: avoid copy for string([]byte) conversion used in string comparison

compiler: avoid copy for string([]byte) conversion used in string comparison

    If a string([]byte) conversion is used immediately in a string
    comparison, we don't need to copy the backing store of the byte
    slice, as the string comparison doesn't hold any reference to
    it. Instead, just create a string header from the byte slice and
    pass it for comparison.

    A new type of expression, String_value_expression, is introduced,
    for constructing string headers.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/170894

	* go.dg/cmpstring.go: New test.

From-SVN: r271021