LoopValStage is already of int.

This patch replaces:

  llvm::copy(Src, std::back_inserter(Dst));

with:

  llvm::append_range(Dst, Src);

for breavity.

One side benefit is that llvm::append_range eventually calls
llvm::SmallVector::reserve if Dst is of llvm::SmallVector.

ModuloScheduleExpanderMVE and ModuloScheduleExpander are used sequentially in 
certain use cases. It is necessary to update live intervals for ModuloScheduleExpanderMVE; 
otherwise, crashes may occur.

Some new registers are reused when replacing some old ones in
certain use case of ModuloScheduleExpander. It is necessary to
avoid repeated interval calculations for these registers.

The interval of newly generated reg in ModuloScheduleExpander is empty.
This will cause crash at some corner case. This patch recalculate the
live intervals of these regs.

After #130165.

In the code: `VRMap[CurStageNum][Def] = VRMap[CurStageNum][LoopVal]`
`VRMap[CurStageNum][LoopVal]` calculates a reference before
`VRMap[CurStageNum][Def]` which may rehash the DenseMap.
Then the reference can be dead.