[llvm] [X86] Add ISD::MULHS/MULHU v4i64/v8i64 lowering (PR #169819)

[via llvm-commits]

Andarwinux wrote:

FYI, UniformConstCostTable and ConstCostTable in X86TargetTransformInfo also need to be updated to add vXi64 UDIV/SDIV costs, otherwise autovectorization still won't work.

Since this will change LV/SLP tests, a third PR may be needed to update the cost model separately.

https://github.com/llvm/llvm-project/pull/169819