[X86] Add test case to show missed opportunity to use a single pmuludq to implement a multiply when a zext lives in another basic block.

This can occur when one of the inputs to the multiply is loop invariant. Though my test cases just use two basic blocks with an unconditional jump which we won't merge until after isel in the codegen pipeline.

For scalars, I believe SelectionDAGBuilder can add an AssertZExt to pass knowledge across basic blocks but its explicitly disabled for vectors.

llvm-svn: 347266

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