PTX: Reverting implementation of i8.

The .b8 operations in PTX are far more limiting than I first thought. The mov operation isn't even supported, so there's no way of converting a .pred value into a .b8 without going via .b16, which is
not sensible. An improved implementation needs to use the fact that loads and stores automatically extend and truncate to implement support for EXTLOAD and TRUNCSTORE in order to correctly support
boolean values.

llvm-svn: 133873