[x86] Fix the new vector shuffle lowering's use of VSELECT for AVX2
lowerings.

This was hopelessly broken. First, the x86 backend wants '-1' to be the
element value representing true in a boolean vector, and second the
operand order for VSELECT is backwards from the actual x86 instructions.
To make matters worse, the backend is just using '-1' as the true value
to get the high bit to be set. It doesn't actually symbolically map the
'-1' to anything. But on x86 this isn't quite how it works: there *only*
the high bit is relevant. As a consequence weird non-'-1' values like
0x80 actually "work" once you flip the operands to be backwards.

Anyways, thanks to Hal for helping me sort out what these *should* be.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218582 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Teach the new vector shuffle lowering to fall back on AVX-512
vectors.

Someone will need to build the AVX512 lowering, which should follow
AVX1 and AVX2 *very* closely for AVX512F and AVX512BW resp. I've added
a dummy test which is a port of the v8f32 and v8i32 tests from AVX and
AVX2 to v8f64 and v8i64 tests for AVX512F and AVX512BW. Hopefully this
is enough information for someone to implement proper lowering here. If
not, I'll be happy to help, but right now the AVX-512 support isn't
a priority for me.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218583 91177308-0d34-0410-b5e6-96231b3b80d8