[AMDGPU] Allow buffer intrinsics to be marked volatile at the IR level (#77847)

In order to ensure the correctness of ptr addrspace(7) lowering, we need
a backwards-compatible way to flag buffer intrinsics as volatile that
can't be dropped (unlike metadata).

To acheive this in a backwards-compatible way, we use bit 31 of the
auxilliary immediates of buffer intrinsics as the volatile flag. When
this bit is set, the MachineMemOperand for said intrinsic is marked
volatile. Existing code will ensure that this results in the appropriate
use of flags like glc and dlc.

This commit also harmorizes the handling of the auxilliary immediate for
atomic intrinsics, which new go through extract_cpol like loads and
stores, which masks off the volatile bit.