[RISCV] Support CONCAT_VECTORS on scalable masks

This patch is a simple fix which registers CONCAT_VECTORS as
custom-lowered for scalable mask vectors. This follows the pattern of
all other scalable-vector types, as the default expansion of
CONCAT_VECTORS cannot handle scalable types, and even if it did it'd go
through the stack and generate worse code.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D103896