[Hexagon] Couple of fixes in optimize addressing mode

[Hexagon] Couple of fixes in optimize addressing mode

- Check if an operand is an immediate before calling getImm. Some operands
  that take constant values can actually have global symbols or other
  constant expressions.
- When a load-constant instruction can be folded into users, make sure to
  only delete it when all users have been successfully converted.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@348802 91177308-0d34-0410-b5e6-96231b3b80d8