[BitcodeReader] Ensure we can read constant vector selects with an i1 condition

Summary:
Constant vectors weren't allowed to have an i1 condition in the
BitcodeReader. Make sure we have the same restrictions that are
documented, not more.

Reviewers: nlewycky, rafael, kschimpf

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12440

llvm-svn: 246459