Further improvements in knc.h

- fix about 94 compfails per llvm version (279 total)
- fix ./tests/ldexp-double.ispc and ./tests/pmuls_vi64.ispc runfails
- add examples/intrinsics/known_fails.txt file to store detailed information about complicated fails
- uncovered 4 runfails (some info about those can be found in 'known_fails.txt'):
 ./tests/funcptr-varying-7.ispc
 ./tests/funcptr-varying-8.ispc
 ./tests/pmulus_vi64.ispc
 ./tests/soa-18.ispc
- remaining fails (runfails + compfails on all llvm versions and optimization levels): 150