[x86] Teach the new vector shuffle lowering to lower v8i32 shuffles with

the native AVX2 instructions.

Note that the test case is really frustrating here because VPERMD
requires the mask to be in the register input and we don't produce
a comment looking through that to the constant pool. I'm going to
attempt to improve this in a subsequent commit, but not sure if I will
succeed.

llvm-svn: 218347