[NFC][OpenMP] Fix worksharing-loop

1. Remove the redundant collapse clause in MLIR OpenMP worksharing-loop
   operation.
2. Fix several typos.
3. Refactor the chunk size type conversion since CreateSExtOrTrunc has
   both type check and type conversion.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D128338