Add lowering of vector dialect to LLVM dialect.

This CL is step 3/n towards building a simple, programmable and portable vector abstraction in MLIR that can go all the way down to generating assembly vector code via LLVM's opt and llc tools.

This CL adds support for converting MLIR n-D vector types to (n-1)-D arrays of 1-D LLVM vectors and a conversion VectorToLLVM that lowers the `vector.extractelement` and `vector.outerproduct` instructions to the proper mix of `llvm.vectorshuffle`, `llvm.extractelement` and `llvm.mulf`.

This has been independently verified to produce proper avx2 code.

Input:
```
func @vec_1d(%arg0: vector<4xf32>, %arg1: vector<8xf32>) -> vector<8xf32> {
  %2 = vector.outerproduct %arg0, %arg1 : vector<4xf32>, vector<8xf32>
  %3 = vector.extractelement %2[0 : i32]: vector<4x8xf32>
  return %3 : vector<8xf32>
}
```

Command:
```
mlir-opt vector-to-llvm.mlir -vector-lower-to-llvm-dialect --disable-pass-threading | mlir-opt -lower-to-cfg -lower-to-llvm | mlir-translate --mlir-to-llvmir | opt -O3 | llc -O3 -march=x86-64 -mcpu=haswell -mattr=fma,avx2
```

Output:
```
vec_1d:                                 # @vec_1d
# %bb.0:
        vbroadcastss    %xmm0, %ymm0
        vmulps  %ymm1, %ymm0, %ymm0
        retq
```
PiperOrigin-RevId: 262895929