[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.

llvm-svn: 368608