IR: Fix a missed case when threading OnlyIfReduced through ConstantExpr

In r216015 I missed propagating `OnlyIfReduced` through the inline
versions of `getGetElementPtr()` (I was relying on compile failures on
mismatches between the header and source signatures to get them all).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216023 91177308-0d34-0410-b5e6-96231b3b80d8