Allow getting the base pointer of an image load/store.

Allow getting the base pointer of an image load/store.

In value numbering, we treat loads and stores of images, ie OpImageLoad,
as a memory operation where it is interested in the "base address" of
the instruction.  In those cases, it is an image instruction.

The problem is that `Instruction::GetBaseAddress()` does not account for
the image instructions, so the assert at the end to make sure it found
a valid base address for its addressing mode fails.

The solution is to look at the load/store instruction to determine how
the assertion should be done.

Fixes #1160.