AMDGPU/GlobalISel: Legalize llvm.amdgcn.s.buffer.load

The 96-bit results need to be widened.

I find the interaction between LegalizerHelper and MIRBuilder somewhat
awkward. The custom legalization is called by the LegalizerHelper, but
then does not have access to the helper. You have to construct a new
helper, which then does not own the MachineIRBuilder, but does modify
it. Maybe custom legalization should be passed the helper?