[X86] Add test case for masked load with constant mask and all zeros passthru.

avx/avx2 masked loads only support all zeros for passthru in hardware.
So we have to emit a blend for all other values. We have an optimization
that tries to optimize this blend if the mask is constant. But we
don't need to perform this optimization if the passthru value is zero
which doesn't need the blend at all.

llvm-svn: 362674