[InstCombine] Fold series of instructions into mull

The following sequence should be folded into in0 * in1
      In0Lo = in0 & 0xffffffff; In0Hi = in0 >> 32;
      In1Lo = in1 & 0xffffffff; In1Hi = in1 >> 32;
      m01 = In1Hi * In0Lo; m10 = In1Lo * In0Hi; m00 = In1Lo * In0Lo;
      addc = m01 + m10;
      ResLo = m00 + (addc >> 32);

Reviewed By: spatel, RKSimon
Differential Revision: https://reviews.llvm.org/D136015