Building on #86248, we can also narrow the width of a mul of zexts.
This is specifically legal because on RVV we always extend to the next
power of 2 width, and multiplying two N bit integers produces a maximum
value of 2\*N bits.
So as long as we keep an inner zext of 2\*N, we will have enough space
for the multiply and won't overflow.
Alive2 proof: https://alive2.llvm.org/ce/z/XteYyb
87519a2830