[RISCV][NFC] Simplify the sp-offset reduction by spimm of CM.PUSH/POP. (#66667)

When inserting prolgue/epilogue, we use the spimm of CM.PUSH/POP to
reduce the following offset for sp. Previously, we tried to use the free
space of the push stack to minimize the following sp-offset. But it's
useless, since the free space must be less than 16 and required stack should
be aligned to 16 before/after the adjustment.