ppc: Allow passing r0 as a second argument to instructions in

PPC_X_OPCODE_D_FORM_LIST.

Change-Id: Ia4fbe3f81bdc8569f7b168c11362e3d99c1d2188
Reviewed-on: https://chromium-review.googlesource.com/1181646Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#55238}

src/ppc/assembler-ppc.h

@@ -660,7 +660,6 @@ class Assembler : public AssemblerBase {
   template <class R>                                                     \
   inline void name(const R rt, const Register ra, const Register rb,     \
                    const RCBit rc = LeaveRC) {                           \
-    DCHECK(ra != r0);                                                    \
     x_form(instr_name, rt.code(), ra.code(), rb.code(), rc);             \
   }                                                                      \
   template <class R>                                                     \