[RISCV] Use reduction result type for EXTRACT_VECTOR_ELT in lowerReductionSeq.

Remove the call to getSExtOrTrunc.

Reduction ISD nodes produce a scalar result and that result is
allowed to be larger than the vector element type due to type
legalization. This is the same rule we allow for EXTRACT_VECTOR_ELT
for the same reason.

We can copy the result type over from the reduction node to
EXTRACT_VECTOR_ELT.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D139757