In D97111 we changed the RVV frame layout when using sp or bp to address the stack slots so we could address the emergency stack slot. The idea is to put the RVV objects as far as possible (in offset terms) from the frame reference register (sp / fp / bp).
When using fp this happens naturally because the RVV objects are already the top of the stack and due to the constraints of RVV (VLENB being a power of two >= 128) the stack remains aligned. The rest of this summary does not apply to this case.
When using sp / bp we need to skip the non-RVV stack slots. The size of the the non-RVV objects is computed subtracting the callee saved register size (whose computation is added in D97111 itself) to the total size of the stack (which does not account for RVV stack slots). However, when doing so we round to 16 bytes when computing that size and we end emitting a smaller offset that may belong to a scalar stack slot (see D98801). So this change removes that rounding.
Also, because we want the RVV objects be between the non-RVV stack slots and the callee-saved register slots, we need to make sure the RVV objects are properly aligned to 8 bytes. Adding a padding of 8 would render the stack unaligned. So when allocating space for RVV (only when we don't use fp) we need to have extra padding that preserves the stack alignment. This way we can round to 8 bytes the offset that skips the non-RVV objects and we do not misalign the whole stack in the way. In some circumstances this means that the RVV objects may have padding before (=lower offsets from sp/bp) and after (before the CSR stack slots).
Unfortunately we are computing the size of the CSR stack slots (CalleeSavedStackSize) in processFunctionBeforeFrameIndicesReplaced which is called after the prologue has been emitted. So we might be less precise here though my expectation is that this is OK because we handle those now as having XLEN size. If this is a problem, then we can unconditionally add enough padding (getStackAlign()) to retain the alignment while allowing us to adjust the area of RVV-objects as needed.