[SLP] Fix order of insertelement/insertvalue seed operands
This patch takes the indices operands of insertelement/insertvalue
into account while generation of seed elements for findBuildAggregate().
This function has kept the original order of inserts before.
Also this patch optimizes findBuildAggregate() preventing it from
redundant temporary vector allocations and its multiple reversing.
Subscribers: hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D83779