[llvm] [LSR] Replace casts with an equivalent std::as_const (NFC) (PR #138980)

[Piotr Fusik via llvm-commits]

https://github.com/pfusik approved this pull request.

LGTM. Replacing C-style casts with C++17 `as_const`, already used in a few places: [StaticAnalyzer](https://github.com/llvm/llvm-project/blob/e7bf75043701119f8aeecff4c203cbcf2266fa62/clang/lib/StaticAnalyzer/Checkers/BlockInCriticalSectionChecker.cpp), [MLIR](https://github.com/llvm/llvm-project/blob/e7bf75043701119f8aeecff4c203cbcf2266fa62/mlir/lib/Dialect/SparseTensor/IR/Detail/DimLvlMapParser.cpp), [MLIR elsewhere](https://github.com/llvm/llvm-project/blob/e7bf75043701119f8aeecff4c203cbcf2266fa62/mlir/lib/Dialect/SparseTensor/IR/Detail/Var.h).

https://github.com/llvm/llvm-project/pull/138980