[llvm] [CostModel][Test] Replace multiple flags with `-intrinsic-cost-strategy` (PR #128885)

[David Sherwood via llvm-commits]

https://github.com/david-arm approved this pull request.

LGTM - thanks for the clean-up! But please wait a day or so in case @fhahn has any objections.

https://github.com/llvm/llvm-project/pull/128885