[mlir][ptr] Add `gather`, `masked_load`, `masked_store`, and `scatter` ops (#156368)

This patch adds the `gather`, `masked_load`, `masked_store`, and
`scatter` operations to the `ptr` dialect. It also implements
translation from these operations to LLVM intrinsics:
- ptr.gather -> llvm.masked.gather
- ptr.masked_load -> llvm.masked.load  
- ptr.masked_store -> llvm.masked.store
- ptr.scatter -> llvm.masked.scatter

Example:
```mlir
llvm.func @mixed_masked_ops_address_spaces(%ptr: !ptr.ptr<#llvm.address_space<3>>, %ptrs: vector<4x!ptr.ptr<#llvm.address_space<3>>>, 
                                          %mask: vector<4xi1>, %value: vector<4xf64>, %passthrough: vector<4xf64>) {
  %0 = ptr.gather %ptrs, %mask, %passthrough alignment = 8 : vector<4x!ptr.ptr<#llvm.address_space<3>>> -> vector<4xf64>
  ptr.scatter %value, %ptrs, %mask alignment = 8 : vector<4xf64>, vector<4x!ptr.ptr<#llvm.address_space<3>>>
  %1 = ptr.masked_load %ptr, %mask, %passthrough alignment = 8 : !ptr.ptr<#llvm.address_space<3>> -> vector<4xf64>
  ptr.masked_store %value, %ptr, %mask alignment = 8 : vector<4xf64>, !ptr.ptr<#llvm.address_space<3>>
  llvm.return
}
```
Translates to:
```llvm
define void @mixed_masked_ops_address_spaces(ptr addrspace(3) %0, <4 x ptr addrspace(3)> %1, <4 x i1> %2, <4 x double> %3, <4 x double> %4) {
  %6 = call <4 x double> @llvm.masked.gather.v4f64.v4p3(<4 x ptr addrspace(3)> %1, i32 8, <4 x i1> %2, <4 x double> %4)
  call void @llvm.masked.scatter.v4f64.v4p3(<4 x double> %3, <4 x ptr addrspace(3)> %1, i32 8, <4 x i1> %2)
  %7 = call <4 x double> @llvm.masked.load.v4f64.p3(ptr addrspace(3) %0, i32 8, <4 x i1> %2, <4 x double> %4)
  call void @llvm.masked.store.v4f64.p3(<4 x double> %3, ptr addrspace(3) %0, i32 8, <4 x i1> %2)
  ret void
}
```

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