[x86] allow more shuffle splitting to avoid vpermps (PR40434)

This is tricky to make optimal: sometimes we're better off using 
a single wider op, but other times it makes more sense to combine
a narrow ops to achieve the same result.

This solves the case from:
https://bugs.llvm.org/show_bug.cgi?id=40434

There's potentially a similar change for vectors with 64-bit elements,
but it needs adjustments similar to rL352333 to avoid creating infinite
loops.

llvm-svn: 352380