MachineIRBuilder: Rename buildMerge. NFC

`buildMerge` may build a G_MERGE_VALUES, G_BUILD_VECTOR or
G_CONCAT_VECTORS. Rename it to `buildMergeLikeInstr`.

This is a follow-up suggested in https://reviews.llvm.org/D140964

Differential Revision: https://reviews.llvm.org/D141372