[Hexagon] Fixes to the bitsplit generation

- Fix the insertion point, which occasionally could have been incorrect.
- Avoid creating multiple bitsplits with the same operands, if an old one
  could be reused.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297414 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed