[Hexagon] Hoist nonnull assert up.

Once a pointer is turned into a reference it cannot be nullptr, clang
rightfully warns about this assert being a tautology. Put the assert
before the reference is created.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@260949 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed