[RISCV] Lower CONCAT_VECTORS to INSERT_SUBVECTOR nodes

The default expansion of CONCAT_VECTORS goes through the stack. This
patch avoids that penalty by custom-lowering CONCAT_VECTORS to a series
of INSERT_SUBVECTOR nodes. Futher optimizations are possible, but this
is a good start.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D97692