SDAG: Legalize vector BSWAP into a shuffle if the shuffle is legal but the bswap not.

- On ARM/ARM64 we get a vrev because the shuffle matching code is really smart. We still unroll anything that's not v4i32 though.
- On X86 we get a pshufb with SSSE3. Required more cleverness in isShuffleMaskLegal.
- On PPC we get a vperm for v8i16 and v4i32. v2i64 is unrolled.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209123 91177308-0d34-0410-b5e6-96231b3b80d8
Flip on vectorization of bswap intrinsics.

The cost model conservatively assumes that it will always get scalarized and
that's about as good as we can get with the generic TTI; reasoning whether a
shuffle with an efficient lowering is available is hard. We can override that
conservative estimate for some targets in the future.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209125 91177308-0d34-0410-b5e6-96231b3b80d8