[llvm] [llvm][CodeGen] avoid repeated interval calculation in window scheduler (PR #132352)

[Hua Tian via llvm-commits]

huaatian wrote:

> Strictly speaking, it might be necessary to fix and restore the following code. The intention of this code is to invalidate the interval because the reference to an existing register was changed. However, the existing register is not `ToReg` but `FromReg`. Additionally, the invalidation should be done using `removeInterval()`. As mentioned above, within the scope currently used by MachinePipeliner, the interval of this register will not be referenced further.

>From the code, it seems that an empty interval is being created for ToReg, which has caused some issues. I think we can remove the interval for FromReg, but it might be more appropriate to implement this in a new PR.

https://github.com/llvm/llvm-project/pull/132352