[AMDGPU] getMemOperandsWithOffset: add vaddr operand for stack access BUF instructions

A consequence is that checkInstOffsetsDoNotOverlap can now distinguish
sp+offset from fp+offset, so it knows that it shouldn't try to work out
whether the accesses overlap just by comparing the offsets. For example
in these two instructions:

MIR:
BUFFER_STORE_DWORD_OFFSET %0:vgpr_32(s32), $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr32, 4, 0, 0, 0, 0, 0, implicit $exec :: (dereferenceable store 4 into stack + 4, addrspace 5)
%4:vgpr_32 = BUFFER_LOAD_DWORD_OFFEN %stack.0.alloca, $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr32, 0, 0, 0, 0, 0, 0, implicit $exec :: (load 4 from `i8 addrspace(5)* undef`, addrspace 5)

ISA:
buffer_store_dword v0, off, s[0:3], s32 offset:4
buffer_load_dword v0, off, s[0:3], s34

Differential Revision: https://reviews.llvm.org/D73957