AMDGPU: Fix handling of div_scale with undef inputs

The src0 register must match src1 or src2, but if these
were undefined they could end up using different implicit_defed
virtual registers. Force these to use one undef vreg or pick the
defined other register.

Also fixes producing invalid nodes without the right number of
inputs when src2 is undef.

llvm-svn: 309743