[LLVM][CodeGen][SVE] Improve lowering of fixed length masked mem ops. (#134402)
Converting fixed length masks, as used by MLOAD, to scalable vectors is
done by comparing the mask to zero. When the mask is the result of a
compare we can instead promote the operands and regenerate the original
compare. At worst this reduces the dependecy chain and in most cases
removes the need for multiple compares.