[GlobalISel] Restrict G_MERGE_VALUES capability and replace with new opcodes.
This patch restricts the capability of G_MERGE_VALUES, and uses the new G_BUILD_VECTOR and G_CONCAT_VECTOR opcodes instead in the appropriate places.
The MIRBuilder is modified to generate G_CONCAT when trying to create MERGE_VALUES of vectors.
This patch also includes AArch64 support for selecting G_BUILD_VECTOR of 4xs32 and 2xs64 vectors.
Could you handle G_BUILD_VECTOR as well? We are unable to legalize the test below because of the missing combine.