[llvm] [HEXAGON] Add AssertSext in sign-extended mpy (PR #149061)

[via llvm-commits]

llvmbot wrote:

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>/cherry-pick [fcabb53](https://github.com/llvm/llvm-project/commit/fcabb53f0c349885167ea3d0e53915e6c42271a7)

Error: Command failed due to missing milestone.

https://github.com/llvm/llvm-project/pull/149061