[GlobalISel] Introduce G_BUILD_VECTOR, G_BUILD_VECTOR_TRUNC and G_CONCAT_VECTOR opcodes.

[GlobalISel] Introduce G_BUILD_VECTOR, G_BUILD_VECTOR_TRUNC 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_BUILD_VECTOR_TRUNC is a special opcode for handling scalar
operands which are larger than the destination vector element type, and
therefore does an implicit truncate.

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.

Differential Revision: https://reviews.llvm.org/D53594

llvm-svn: 348430