Fix a severe compile time problem when forming large SCEV expressions.

This fix is very lightweight. The same fix already existed for AddRec
but was missing for NAry expressions.

This is obviously an improvement and I'm unsure how to test compile
time problems.

Patch by Xiaoyi Guo!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187475 91177308-0d34-0410-b5e6-96231b3b80d8