AMDGPU: Correct encoding for global instructions

The soffset field needs to be be set to 0x7f to disable it,
not 0. 0 is interpreted as an SGPR offset.

This should be enough to get basic usage of the global instructions
working. Technically it is possible to use an SGPR_32 offset,
but I'm not sure if it's correct with 64-bit pointers, but
that is not handled now. This should also be cleaned up
to be more similar to how different MUBUF modes are handled,
and to have InstrMappings between the different types.

llvm-svn: 308583