[Codegen][LLVM] Accept splat form in VLA broadcast test

[tlopex]

Newer LLVM versions (observed with LLVM 20) print a scalable broadcast store as a splat constant, e.g.
store <vscale x 4 x float> splat (float 1.000000e+00), instead of the older shufflevector (<vscale x 4 x float> insertelement (... form. Accept either representation in test_scalable_broadcast so the test passes across LLVM versions while still verifying scalable vector codegen.

[gemini-code-assist]

Code Review

This pull request updates the LLVM VLA codegen test to support newer LLVM versions by allowing the broadcast assertion to match either a shufflevector pattern or a splat constant pattern. The reviewer suggested a performance and readability improvement to use the Python in operator instead of re.findall for literal string matching.

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[gemini-code-assist]

Since both patterns are literal strings without any dynamic regex patterns (like wildcards or character classes), we can use the Python in operator instead of re.findall. This is more readable, avoids the need to escape parentheses and dots, and is significantly more efficient.

    assert (
        "shufflevector (<vscale x 4 x float> insertelement (<vscale x 4 x float>" in llvm
        or "store <vscale x 4 x float> splat (float 1.000000e+00)" in llvm
    ), "No scalable broadcast in generated LLVM."

[tqchen]

please adopt gemini reviews