[X86][SSE] Move VZEXT_MOVL removal into SimplifyDemandedVectorEltsForTargetNode

This patch replaces the VZEXT_MOVL removal from combineShuffle with a more general version based in SimplifyDemandedVectorEltsForTargetNode.

By using computeKnownBits we can always remove the VZEXT_MOVL if the upper elements of the source operand are known to be zero.

This requires us to add the conversion ops to computeKnownBitsForTargetNode as well.

Reviewed By: @craig.topper

Differential Revision: https://reviews.llvm.org/D79335