improvements for the stack cache analysis pass

The pass has moved to the pre-emit stage in order to rely upon the final
instruction order. This is irrelevant for the SC analysis itself, but not when
we use instruction indexes to match its results with PML instructions.
ensure analysis results now exported via PML

Some general changes wrt. SC analysis:
* the analysis runs after the function splitter
* the analysis results are stored in a pseudo pass (see
  PatmosStackCacheAnalysis.h) and used by PML export
* optimizing (removing) ensures needs to enabled: -mpatmos-sca-remove-ensures

Platin's pml2ais exports the ensure fill count into an .ais annotation.