Micro-optimization: don't shift an entire bitcode record over to get the code.

Previously, BitstreamCursor read an abbreviated record by splatting the
whole thing into a data vector, then extracting and removing the /first/
element. Now, it reads the first element--the record code--separately from
the actual field values.

No (intended) functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181639 91177308-0d34-0410-b5e6-96231b3b80d8