[X86][SSE] combinePMULDQ - pmuldq(x, 0) -> zero vector (PR43159)

ISD::isBuildVectorAllZeros permits undef elements to be present, which means we can't return it as a zero vector. PMULDQ/PMULUDQ is an extending multiply so a multiply by zero of the lower 32-bits should result in a zero 64-bit element.

llvm-svn: 370404