[AMDGPU] Add MMOs for GFX11 Streamout Instructions

The GFX11 NGG Streamout Instructions perform atomic operations on
dedicated registers. At the moment, they lack machine memory operands,
which causes the si-memory-legalizer pass to treat them conservatively
and introduce several unnecessary waits and cache invalidations.

This patch introduces a new address space to represent these special
registers and teaches instruction selection to add memory operands with
this new address space to DS_ADD/SUB_GS_REG_RTN.

Since this address space is meant to be compiler-internal, we move it
up a bit from the other address spaces and give it the number 128.
According to the LLVM Language Reference, address space numbers can go
all the way up to 2^24, but I'm not sure how well this is supported in
practice [1], so using a smaller number seems safer.

[1] https://github.com/llvm/llvm-project/blob/0107513fe79da7670e37c29c0862794a2213a89c/llvm/utils/TableGen/IntrinsicEmitter.cpp#L401

Differential Revision: https://reviews.llvm.org/D146031