Revert "[Hexagon] Don't build two halves of HVX vector in parallel"

This reverts commit ba07f300.

A build-vector sequence is made of pairs: rotate+insert. When constructing
a single vector, this results in a chain of 2*N instructions. The rotate
operation is a permute operation, but the insert uses a multiplication
resource: insert and rotate can execute in the same cycle, but obviously
they cannot operate on the same vector. The original halving idea is still
beneficial since it does allow for insert/rotate overlap, and for hiding
insert's latency.