compiler: inline functions with assignments and return statements

compiler: inline functions with assignments and return statements

    Support inlining functions that contain only assignments and return
    statements, with expressions of either constants or parameters.
    Functions that contain other kinds of statements or expressions are
    not yet inlined.  With this change, about 100 functions in the
    standard library are inlinable.

    Reviewed-on: https://go-review.googlesource.com/c/150073

From-SVN: r266573