libclc: Add assembly versions of vload for global int4/8/16

The assembly should be generic, but at least currently R600 only supports
32-bit loads of int1/4, and I believe that only global is well-supported.

R600 lowers the 8/16 component vectors to multiple 4-bit loads.

The unoptimized C versions of the other stuff is left in place.

Patch by: Aaron Watry

llvm-svn: 185008