Add bitselect() builtin

Reviewed-By: Aaron Watry <awatry@gmail.com>
Implement barrier() builtin

Reviewed and Tested-by: Aaron Watry <awatry@gmail.com>
Add __CLC_ prefix to all macro definitions in headers

libclc was defining and undefing GENTYPE and several other macros with
common names in its header files.  This was preventing applications from
defining macros with identical names as command line arguments to the
compiler, because the definitions in the header files were masking the
macros defined as compiler arguements.

Reviewed-by: Aaron Watry <awatry@gmail.com>
Implement mad24() and mul24() builtins

Reviewed-by: Aaron Watry <awatry@gmail.com>
PPC: Mark vector FREM as Expand by default

Another bug found by llvm-stress! This fixes crashing with:
  LLVM ERROR: Cannot select: v4f32 = frem ...