[SVE][AArch64TTI] Fix invalid mla combine that miscomputes the value of inactive...

[SVE][AArch64TTI] Fix invalid mla combine that miscomputes the value of inactive lanes.

Consider: add(pg, a, mul_u(pg, b, c))

Although the multiply's inactive lanes are undefined, they don't
contribute to the final result.  The overall result of the inactive
lanes come from "a" and thus the above is another form of mla
rather than mla_u.