This patch substantially simplifies and cleans up handling of basic blocks

This patch substantially simplifies and cleans up handling of basic blocks
in the bytecode parser.  Before we tried to shoehorn basic blocks into the
"getValue" code path with other types of values.  For a variety of reasons
this was a bad idea, so this patch separates it out into its own data structure.

This simplifies the code, makes it fit in 80 columns, and is also much faster.
In a testcase provided by Bill, which has lots of PHI nodes, this patch speeds
up bytecode parsing from taking 6.9s to taking 2.32s.  More speedups to
follow later.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@8977 91177308-0d34-0410-b5e6-96231b3b80d8