Change all of the bytecode reader primitives to throw exceptions instead of

Change all of the bytecode reader primitives to throw exceptions instead of
returning error codes.  Because they don't return an error code, they can
return the value read, which simplifies the code and makes the reader more
efficient (yaay!).

Also eliminate the special case code for little endian machines.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10871 91177308-0d34-0410-b5e6-96231b3b80d8