[X86] Fix checks for illegal physreg COPY instructions

D105263 changed this test to not expect a MachineVerifier error on this
instruction:

    ; FP16 reg is sub_reg of xmm
    %0:_(s16) = COPY $xmm0

D107082 changed the behaviour back again so that this instruction did
cause an error, but the test was not updated to expect the error.

Differential Revision: https://reviews.llvm.org/D148534