[RISCV] Teach lowerScalarInsert to handle scalar value is the first element of a fixed vector.

D155929 teach lowerScalarInsert to handl start value (extractelement scalable_vector, 0)
and specifically converts fixed extracted vectors to scalable vectors when
lowering vector reduction. It's not enough because there is another way to
create (extractelement fixed_vector, 0) as a start value of lowerScalarInsert
like #64327.

#64327: https://github.com/llvm/llvm-project/issues/64327.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D156863