[AMDGPU][GlobalISel] Add RegBankLegalize support for G_PTRMASK (#174889)
Add support for G_PTRMASK but we are missing p8 (buffer resource) due to
a legalizer issue in GlobalISel which does not occur on SelectionDAG:
`LLVM ERROR: unable to legalize instruction: %17:_(p8) = G_PTRMASK %0:_,
%22:_(s128) (in function: v_ptrmask_buffer_resource_variable_i48)`
Added a FIXME to indicate this issue.