While loop stucks , freezes !

Hello ..

I am having a very weird issue with my code piece ..
The MCU is STM32F407VET6 .. What I'm trying to do is sending a string via USART1 peripheral module .. In order check if the character in the USART1 is sent out of the USART1->DR register I am continuously checking the "Transmit Complete Bit of the usart1 status register
The code below works well ...

#define TC1     *((uint32_t *)(0x42220018))    //Previously defined at
                                               //the top of the main file
void EXTI4_IRQHandler(void)
{
        uint8_t i;
        char x[6]="BURAK\n";
        EXTI_PR_Bit4=1; // Cleared by writing 1
    // ISR code
        TC1=0;
        for(i=0;i<7;i++)
        {
                USART1->DR=x[i];
                while(!(USART1->SR&USART_SR_TC));//      Transmit Complete ?
        }
        TC1=0;
}

BUT !

When I replace

while(!(USART1->SR&USART_SR_TC));

with

while(TC==0);

The code stucks.. freezes ... and stops ...Why does it happen ?? Are Bit-Band Aliases not reliable things to use with while ,if , for statements ?? Does it have to be read/modify/write sequence always ??

Thanks in advance ..