Bitcode: Simplify BitstreamWriter::EnterBlockInfoBlock() interface.

No block info block should need to define local abbreviations, so we can
always use a code width of 2.

Also change all block info block writers to use EnterBlockInfoBlock.

Differential Revision: https://reviews.llvm.org/D26168

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@285660 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed