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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@18685 91177308-0d34-0410-b5e6-96231b3b80d8