[BOLT][NFC] Fix UB due to left shift of negative value

[jobnoorman]

Summary

The following test fails when enabling UBSan due to a left shift of a
negative value:

runtime error: left shift of negative value -2

BOLT :: AArch64/ext-island-ref.s

This patch fixes this by using a multiplication instead of a shift.

[yota9]

LGTM

bolt/lib/Target/AArch64/AArch64MCPlusBuilder.cpp

for (unsigned I = 0, E = MCII.getNumOperands(); I != E; ++I) {
 
       if (!Inst.getOperand(I).isImm()) {
         assert(Inst.getOperand(I).isExpr() && "Unexpected PCREL operand");
         assert(DispExpr && "DispExpr must be set");
         *DispExpr = Inst.getOperand(I).getExpr();
         return true;
       }
 
-      DispImm = Inst.getOperand(I).getImm() << 2;
+      DispImm = Inst.getOperand(I).getImm() * 4;
       return true;
     }
     return false;
   }
 
   bool evaluateMemOperandTarget(const MCInst &Inst, uint64_t &Target,
                                 uint64_t Address,
                                 uint64_t Size) const override {