TargetLowering: Change isShuffleMaskLegal's mask argument type to ArrayRef<int>. NFCI.

Changing mask argument type from const SmallVectorImpl<int>& to
ArrayRef<int>.

This came up in D35700 where a mask is received as an ArrayRef<int> and
we want to pass it to TargetLowering::isShuffleMaskLegal().
Also saves a few lines of code.

llvm-svn: 309085