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

[Hua Tian via llvm-commits]

huaatian wrote:

@arsenm Do you think it is appropriate to not calculate the register's live interval initially, but instead delay the calculation until getInterval() is called, as suggested by @ytmukai？
https://github.com/llvm/llvm-project/pull/132677#discussion_r2012106361

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