GlobalISel: convert buildSequence to use non-deprecated instructions.

GlobalISel: convert buildSequence to use non-deprecated instructions.

G_SEQUENCE is going away soon so as a first step the MachineIRBuilder needs to
be taught how to emulate it with alternatives. We use G_MERGE_VALUES where
possible, and a sequence of G_INSERTs if not.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306119 91177308-0d34-0410-b5e6-96231b3b80d8