[X86][BtVer2] Limit zero idiom tests to a single iteration.

Reduces output size and we're only wanting to check that the instructions are fast-path'd (just Dispatch+Retire) anyhow

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334292 91177308-0d34-0410-b5e6-96231b3b80d8

Author: RKSimon