[X86] Make the multiply and divide itineraries more consistent.

Sometimes we used the same itinerary for MEM and REG forms, but that seems inconsistent with our usual usage.

We also used the MUL8 itinerary for MULX32/64 which was also weird.

The test changes are because we were using IIC_IMUL32_RR and IIC_IMUL64_RR instead of IIC_IMUL32_REG/IIC_IMUL64_REG for the 32 and 64 bit multiplies that produce double width result.

llvm-svn: 327866