[NFC] Use opaqueptr in llvm/test/Transforms/SimplifyCFG/preserve-llvm-loop-metadata.ll

Use opaqueptr for test case
llvm/test/Transforms/SimplifyCFG/preserve-llvm-loop-metadata.ll.

- Adjust variable number accordingly since bitcast between different pointer
  types are not necessary.

Differential Revision: https://reviews.llvm.org/D134159