[RISCV] Remove one use restriction on the scalar from combineBinOpToReduce.

If multiple reductions use the neutral start value, this prevented
us from optimizing any of them.

This increases the number of scalar->vector operations but reduces
the number of scalar operations.

Note, we were already inconsistent about the one use check when we
peeked through INSERT_SUBVECTORs.

Reviewed By: fakepaper56

Differential Revision: https://reviews.llvm.org/D149181