[x86] move shrunkblend transform to helper function; NFCI

We should be able to obsolete D48043 by easing the constraints
on this existing code. 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334504 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed