X86: add some missing lowerings for shuffles on `bf16` element type. (#76076)

Some shuffles with `bf16` as element type were running into a
`llvm_unreachable`. Key to reproducing was to chain two shuffles.

```llvm
define <2 x bfloat> @shuffle_chained_v32bf16_v2bf16(<32 x bfloat> %a) {
  %s = shufflevector <32 x bfloat> %a, <32 x bfloat> zeroinitializer, <32 x i32> <i32 0, i32 16, i32 1, i32 17, i32 2, i32 18, i32 3, i32 19, i32 4, i32 20, i32 5, i32 21, i32 6, i32 22, i32 7, i32 23, i32 8, i32 24, i32 9, i32 25, i32 10, i32 26, i32 11, i32 27, i32 12, i32 28, i32 13, i32 29, i32 14, i32 30, i32 15, i32 31>
  %s2 = shufflevector <32 x bfloat> %s, <32 x bfloat> zeroinitializer, <2 x i32> <i32 0, i32 1>
  ret <2 x bfloat> %s2
}
```

This was hitting this UNREACHABLE:

```
Not a valid 512-bit x86 vector type!
UNREACHABLE executed at /home/benoit/iree/third_party/llvm-project/llvm/lib/Target/X86/X86ISelLowering.cpp:17124!
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.      Program arguments: /home/benoit/mlir-build/bin/llc -mtriple=x86_64-unknown-linux-gnu -mattr=+avx512f,+avx512vl,+avx512bw,+avx512bf16
1.      Running pass 'Function Pass Manager' on module '<stdin>'.
2.      Running pass 'X86 DAG->DAG Instruction Selection' on function '@shuffle_chained_v32bf16_v2bf16'
```