Eliminate this ugly hack. This was put back in when replaceAllUsesOf used

a different algorithm that was extremely inefficient for instructions with
many operands.

This reduces the time of this code snippet from .23s for 176.gcc to 0.03s
in a debug build, which speeds up total llvm-dis time just barely.

It's more of a code cleanup than a speedup.

llvm-svn: 18685