[TargetLowering] Expand the last stage of i16 popcnt using shift+add+and instead of mul+shift.

If we use multiply it would be with 0x0101 which is 1 more than a power
of 2. On some targets we would expand this to shl+add. By avoiding the
multiply earlier, we can generate better code.

Note, PowerPC doesn't do the shl+add expansion of multiply so one of
the tests increased in instruction count.

Limiting to scalars because it almost always increased the number of
instructions in vector tests.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D125638