Introduce G_BUILD_VECTOR and G_CONCAT_VECTOR opcodes.
These opcodes are intended to subsume some of the capability of G_MERGE_VALUES, as it was too powerful and thus complex to add deal with throughout the GISel pipeline.
G_BUILD_VECTOR creates a vector value from a sequence of uniformly typed scalar values.
G_CONCAT_VECTOR creates a vector by concatenating smaller, uniformly typed, vectors together.
These will be used in a subsequent commit. This commit just adds the initial infrastructure.