MachineIRBuilder: Add buildMergeValues. NFC

Add a `buildMergeValues` method that unconditionally builds a
G_MERGE_VALUES instruction, as opposed to `buildMergeLikeInstr` which
may decide on a different opcode based on the input types.

I haven't audited all the uses of `buildMergeLikeInstr` to see if they
can be replaced with `buildMergeValues`, but I did find a couple of
obvious ones where we check that we're merging scalars right before
calling `buildMerge`.

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

Differential Revision: https://reviews.llvm.org/D141373