[X86][SSE] Enable post-legalize vXi64 shuffle combining on 32-bit targets

Long ago (2010 according to svn blame), combineShuffle probably needed to prevent the accidental creation of illegal i64 types but there doesn't appear to be any combines that can cause this any more as they all have their own legality checks.

Differential Revision: https://reviews.llvm.org/D30213

llvm-svn: 296966