[RISCV] Use the first element of source as the start value of reduction.

Previously when llvm.reduce.* lowered, riscv backend created scalar vector with
netural element as start value. For llvm.reduce.and/or/min/max/fmax/fmin, we
could use the first element of source as the start value. It's benefit for RVV
since we could just use source vector as start vector.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D155929