[PowerPC] Improve the way legalize mul for v8i16 and add pattern to match mul + add

We can legalize the operation MUL for v8i16 with instruction (vmladduhm A, B, 0)
if altivec enabled. Now, it is set as custom and expand it later, which is not
the right way. And then, we can add the pattern to match the mul + add with (vmladduhm A, B, C)

Reviewed By: Nemanjai

Differential Revision: https://reviews.llvm.org/D76751