[PowerPC] Cleanly reject asm crbit constraint with -crbits

When crbits are disabled, cleanly reject the constraint (return the register
class only to cause an assert later).

llvm-svn: 251566
diff --git a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
index b1f8783..ce6f3a6 100644
--- a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -10927,7 +10927,8 @@
     case 'y':   // crrc
       return std::make_pair(0U, &PPC::CRRCRegClass);
     }
-  } else if (Constraint == "wc") { // an individual CR bit.
+  } else if (Constraint == "wc" && Subtarget.useCRBits()) {
+    // An individual CR bit.
     return std::make_pair(0U, &PPC::CRBITRCRegClass);
   } else if (Constraint == "wa" || Constraint == "wd" ||
              Constraint == "wf") {