InstCombine: Fix another infinite loop caused by visitFPTrunc

We would attempt to replace an frem's operand with the same operand.
This would cause InstCombine to think real work was done, causing
InstCombine to enter an infinite loop.

This fixes the second part of PR21576.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222265 91177308-0d34-0410-b5e6-96231b3b80d8