[X86] Add a paranoia type check to the code that detects AVG patterns from truncating...

[X86] Add a paranoia type check to the code that detects AVG patterns from truncating stores.

If we're after type legalize, we should make sure we won't create
a store with an illegal type when we separate the AVG pattern
from the truncating store.

I don't know of a way to fail for this today. Just noticed while
I was in the vicinity.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368608 91177308-0d34-0410-b5e6-96231b3b80d8