The 16-bit VAddr arguments to A16 image instructions are packed into
legal VGPR_32 operands in AMDGPULegalizerInfo::legalizeImageIntrinsic on
all subtargets. With True16, we also need to pack if the number of VAddr is one
because VGPR_16 is not a legal argument to those Image instructions.
No change to emitted code intended on subtargets pre-GFX11, and none on GFX11
until True16 is active.
A comment explaining why it's important to include the case of NumVAddrs == 1 might be helpful.