compiler: permit inlining constant expressions and expression statements

compiler: permit inlining constant expressions and expression statements

    This relatively minor change increases the number of inlinable
    functions/methods in the standard library from 983 to 2179.

    In particular it permits inlining math/bits/RotateLeftNN.  This
    restores the speed of crypto/sha256 back to what it was before the
    update to 1.13beta1.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/194340

From-SVN: r275558