compiler: permit inlining temporary statements and references

compiler: permit inlining temporary statements and references

    This increases the number of inlinable functions from 439 to 455.
    An example is math/bits.Mul32, which uses temporaries to handle the
    tuple assignment.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/180837

From-SVN: r272022