PMLExport: dump symbolic header bounds

For predictable loops, PMLExport dumps the symbolic header bounds
it can obtain from scalar evolution.
We might want a more structured/processed approach as soon as we
come up with a sensible format.
PatmosExport: formal argument - register mappings

The mappings are computed in a target specific way.
Most of the code was copied from SelectionDAGBuilder to compute
lowered argument types with the correct flags.

Currently, VarArg functions are not supported.
The proper yaml output has yet to be done.
PML/PatmosExport: proper output of arg-reg mapping
PMLExport: added stats counter for loop bounds
PatmosExport: Fix crash args are on stack

* crashed when not vararg but arguments are on stack
* printing changed to assembly reg names
PMLExport: don't print symbolic bounds if constant

* handled by const header bound
PMLExport: symbolic bounds are optional