R600/SI: Convert v16i8 resource descriptors to i128

Now that compute support is better on SI, we can't continue using v16i8
for descriptors since this is also a legal type in OpenCL.

This patch fixes numerous hangs with the piglit OpenCL test and since
we now use a target specific DAG node for LOAD_CONSTANT with the
correct MemOperandFlags, this should also fix:

https://bugs.freedesktop.org/show_bug.cgi?id=66805

llvm-svn: 188429