[mips][FastISel] Allow computation of addresses from constant expressions.

[vkalintiris]

Summary

Try to compute addresses when the offset from a memory location is a constant expression.

[dsanders]

Could you add a test case?

[vkalintiris]

Added a simple test-case.

[vkalintiris]

Modified the commit message to better reflect the change introduced with this patch.

[dsanders]

LGTM

lib/Target/Mips/MipsFastISel.cpp

bool MipsFastISel::computeAddress(const Value *Obj, Address &Addr, Type *Ty) {
   if (const Instruction *I = dyn_cast<Instruction>(Obj)) {
     // Don't walk into other basic blocks unless the object is an alloca from
     // another block, otherwise it may not have a virtual register assigned.
     if (FuncInfo.StaticAllocaMap.count(static_cast<const AllocaInst *>(Obj)) ||
         FuncInfo.MBBMap[I->getParent()] == FuncInfo.MBB) {
       Opcode = I->getOpcode();
       U = I;
     }
-  } else if (isa<ConstantExpr>(Obj))
-    return false;
+  } else if (const ConstantExpr *C = dyn_cast<ConstantExpr>(Obj)) {
+    Opcode = C->getOpcode();
+    U = C;
+  }
   switch (Opcode) {
   default:
     break;
   case Instruction::BitCast: {
     // Look through bitcasts.
     return computeAddress(U->getOperand(0), Addr, Ty);
   }
   case Instruction::GetElementPtr: {