[AMDGPU][CodeGen] Update support (soffset + offset) s_buffer_load's (#68302)

getBaseWithConstantOffset() is used for scalar and non-scalar buffer
loads. Diffrence between s_load and load instruction is that s_load
instruction extends 32-bit offset to 64-bits, so a 32-bit (address +
offset) should not cause unsigned 32-bit integer wraparound, because it
performs addition in 64-bits.