A/D conversion loop infinitly
One way I know of acknowledging whether the conversion is complete is reading the GO/DONE bit of ADCON0. The bit goes low once the conversion is complete. So you can use

ADCON0bits.GO=1;
while(ADCON0bits.GO);


If you still want to go with ADIF, you should make sure you have enabled the ADC interrupt. Also check whether the oscillator pins have some voltage on them or whether it is soldered well. The voltage should be around 2.3V to 3V, not exactly but a stable value when measured with DC voltmeter.

Good Luck.