compiler: improve escape analysis on interface conversions

compiler: improve escape analysis on interface conversions

    If an interface does not escape, it doesn't need a heap
    allocation to hold the data (for non-direct interface type).
    This CL improves the escape analysis to track interface
    conversions, and reduces these allocations.

    Implicit interface conversions were mostly added late in the
    compilation pipeline, after the escape analysis. For the escape
    analysis to see them, we move the introduction of these
    conversions earlier, right before the escape analysis.

    Now that the compiler can generate interface conversions inlined,
    gcc/testsuite/go.test/test/nilptr2.go needs to be adjusted as in
    golang.org/cl/176579, so the use function does an actual use.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/176459

	* go.test/test/nilptr2.go: Change use function to actually do
	something.

From-SVN: r271276