AMDGPU: Use IsSSA property check instead of asserting on isSSA

AMDGPU: Use IsSSA property check instead of asserting on isSSA

Also fix an SSA violation in a test the MIRParser/verifier fails to
catch. It's illegal to define a subregister in SSA. For the purpose of
the test, it just needs to define the super-register to use the
subregister in the use operand.