[RegisterBankInfo] Allow to set a register class when nothing else is set

This is going to be needed to be able to constraint register class on
target specific instruction while the RegBankSelect pass did not run
yet.

llvm-svn: 290375

Author: Quentin Colombet

llvm/lib/CodeGen/GlobalISel/RegisterBankInfo.cpp

@@ -224,12 +224,11 @@ const TargetRegisterClass *RegisterBankInfo::constrainGenericRegister(
     return MRI.constrainRegClass(Reg, &RC);
 
   const RegisterBank *RB = RegClassOrBank.get<const RegisterBank *>();
-  assert(RB && "Generic register does not have a register bank");
-
   // Otherwise, all we can do is ensure the bank covers the class, and set it.
-  if (!RB->covers(RC))
+  if (RB && !RB->covers(RC))
     return nullptr;
 
+  // If nothing was set or the class is simply compatible, set it.
   MRI.setRegClass(Reg, &RC);
   return &RC;
 }