If save/restore libcall or Zcmp push/pop are enabled, callee-saved registers including ra would be
assigned a negative frame index (-1 for ra, -2 for s0, ..., -13 for s11) by RISCVRegisterInfo::hasReservedSpillSlot.
getLibCallID would find a callee-saved register that has max register id and with assigned negative frame index.
And use this max register to get which save/restore libcall should be selected.
In getMaxPushPopReg (for Zcmp push/pop, similar getLibCallID), it uses extra register class (PGPR) to
check the saved register is in ra, s0-s11 instead of checking the frame index is negative.
This patch just changes the checking for the saved register from is contained in PGPR to has a negative frame index.