compiler: Add fission-for-pressure subpass

[FabioLuporini]

So far, this isn't enabled by default. More experimentation required

This PR also adds a new compiler flag to GNUCompiler when running on Skylake and later processors. I've tested it in a few places, and it seems to generally improve things. I'd like people to reproduce it. Note that you need to ensure DEVITO_PLATFORM is properly set to skx, clk, ... the autodetection mechanism, with the recent proliferation of intel architectures, doesn't seem to be as good as it was.