[llvm] [LoongArch] Add LASX sext_invec pattern for v16i8 to v4i64 (PR #219350)

[via llvm-commits]

lrzlin wrote:

`v4i64 = sign_extend_vector_inreg v8i16` will also cause this error, the https://github.com/llvm/llvm-project/pull/219351 should have a more comprehensive coverage.

https://github.com/llvm/llvm-project/pull/219350