Neatened PTX parser and lexer. Added (untested) support for predicated execution.
Added PTX "setp" and "bra" instructions. Ruined "op" module slightly by putting some PTX-specific stuff in there. Added boolean registers (p0, p1, ...).  PTX instructions can now be labelled. Added "s64" register type (had been missing). Now I think all of Daniel's PTX litmus tests are accepted by Herd -- hooray!
Merge branch 'master' of https://github.com/herd/herdtools