[CodeGen] Don't crash classifying a union of an AVX vector and an int

We forgot to run postMerge after decided that the union had to be
classified as MEMORY.  This left us with Lo == MEMORY and Hi == SSEUp
which is an invalid combination.

This fixes PR24021.

Differential Revision: http://reviews.llvm.org/D10908

llvm-svn: 241666