[RISCV] Fix crash when trying to remove segment (#146524)
LiveInterval DefLI can be consisting of multiple segments, and SlotIndex
NewDefSI can be inside any of them. Currenty it is assumed that NewDefSI
belongs to the first segment, and when trying to call removeSegment,
clang crashes since there is no segment containing [DefLI.beginIndex(),
NewDefSI]
FIxes https://github.com/llvm/llvm-project/issues/146518
