[DAGCombiner] loosen restrictions for moving shuffles after vector binop

There are 3 changes to make this correspond to the same transform in instcombine:
1. Remove the legality check - we can't create anything less legal than we started with.
2. Ease the use restriction, so we only bail out if both operands have >1 use.
3. Ease the use restriction for binops with a repeated operand (eg, mul x, x).

As discussed in D60150, there's a scalarization opportunity that will be made
easier by allowing this transform more generally.

llvm-svn: 357580