Cleanup rocMLIR build warnings

[justinrosner]

Motivation

This is just a small PR to clean up build warnings that happen when building rocMLIR.

Technical Details

• rocmlirCustomTosaDecompose: Switch from rewriter.create<tensor::EmptyOp> -> tensor::EmptyOp::create
• AmdArchDb: .Cases("a", "b", val) → .Cases({"a", "b"}, val)
• EmulateNarrowType: Replace rewriter.create calls with op::create

Test Plan

• PR CI

Test Result

• PR CI

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.

[Copilot]

Pull request overview

This PR cleans up rocMLIR build warnings by updating a few MLIR/LLVM API call sites to their non-deprecated forms.

Changes:

• Replace several rewriter.create<...> uses with Op::create(...) helpers in Rock narrow-type emulation and rocMLIR custom TOSA decompose.
• Update llvm::StringSwitch usage to the non-deprecated Cases({ ... }, value) overload for gfx10 minor parsing.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
mlir/lib/Dialect/Rock/Transforms/EmulateNarrowType.cpp	Switch constant and reinterpret_cast construction to Op::create helpers.
mlir/lib/Dialect/Rock/IR/AmdArchDb.cpp	Use StringSwitch::Cases(std::initializer_list<...>) to avoid deprecated overload warnings.
mlir/lib/Conversion/RocmlirCustomTosaDecompose/RocmlirCustomTosaDecompose.cpp	Construct tensor.empty and tensor.insert_slice via Op::create helpers.

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