AMDGPU/GlobalISel: Fix mapping G_ICMP with constrained result

When SI_IF is inserted, it constrains the source register with a
register class, which was quite likely a G_ICMP. This was incorrectly
treating it as a scalar, and then applyMappingImpl would end up
producing invalid MIR since this was unexpected.

Also fix not using all VGPR sources for vcc outputs.