correct mistake in muladd function logic (#2269)
correct mistake in muladd function logic (#2269)

Doesn't make sense to have %1 = mul(%x, %y) computed but never use the result %1
dmlc/tvm,Huyuwei/tvm,Huyuwei/tvm,Laurawly/tvm-1,sxjscience/tvm,Laurawly/tvm-1,dmlc/tvm,Laurawly/tvm-1,tqchen/tvm,Huyuwei/tvm,dmlc/tvm,dmlc/tvm,sxjscience/tvm,Laurawly/tvm-1,Laurawly/tvm-1,tqchen/tvm,Huyuwei/tvm,sxjscience/tvm,tqchen/tvm,dmlc/tvm,sxjscience/tvm,sxjscience/tvm,sxjscience/tvm,dmlc/tvm,Huyuwei/tvm,Huyuwei/tvm,sxjscience/tvm,dmlc/tvm,dmlc/tvm,sxjscience/tvm,dmlc/tvm,Laurawly/tvm-1,Huyuwei/tvm,Laurawly/tvm-1,sxjscience/tvm,Laurawly/tvm-1,Laurawly/tvm-1,Huyuwei/tvm,tqchen/tvm,tqchen/tvm,tqchen/tvm,Huyuwei/tvm,Laurawly/tvm-1,tqchen/tvm,tqchen/tvm,tqchen/tvm,tqchen/tvm