lowp: add move_src_dst and srcover

This is enough to run the bench SkRasterPipeline_compile.

    $ ninja -C out monobench; and out/monobench SkRasterPipeline_compile 300

    Before: 300  SkRasterPipeline_compile 48.4858ns
    After:  300  SkRasterPipeline_compile 37.5801ns

Change-Id: Icb80348908dfb016826700a44566222c9f7a853c
Reviewed-on: https://skia-review.googlesource.com/18595
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed