IR: Split up Constant{Array,Vector}::get(), NFC

Introduce `getImpl()` that tries the simplification logic from `get()`
and then gives up.  This allows the logic to be reused elsewhere in a
follow-up commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215963 91177308-0d34-0410-b5e6-96231b3b80d8