[X86] Don't look for TEST instruction shrinking opportunities when the root node...

[X86] Don't look for TEST instruction shrinking opportunities when the root node is a X86ISD::SUB.

I don't believe we ever create an X86ISD::SUB with a 0 constant which is what the TEST handling needs. The ternary operator at the end of this code shows up as only going one way in the llvm-cov report from the bots.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@324865 91177308-0d34-0410-b5e6-96231b3b80d8