Fix an issue of SPV generation for imageAtomicStore.

Fix an issue of SPV generation for imageAtomicStore.

For GLSL function imageAtomicStore, it will be translated to
OpImageTexelPointer + OpAtomicStore. The result type of
OpImageTexelPointer must be the same as the sampled type of OpTypeImage.
On translation, the result type is mistakenly fetched from operand list
operands[2] while operands[2] corresponds to sampleNum whose type is
always uint. This leads to an error if the image type is iimageXXX that is
int image.