Remove the IsStreamed member variable.

Remove the IsStreamed member variable.

Having different code paths for streamed and regular bitcode reading was a
source of bugs in the past and this defines them away.

It has a small but noticeable impact on performance. I timed running
"opt -disable-output -disable-verify" on a ltoed clang. It goes from

14.752845231 seconds time elapsed   ( +-  0.16% )

to

15.012463721 seconds time elapsed   ( +-  0.11% )

Extracted from a patch by Karl Schimpf.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240305 91177308-0d34-0410-b5e6-96231b3b80d8