ac/nir: Improve integer gather workaround

This fixes one of the few remaining areas where the radeonsi NIR path was missing something that TGSI has. Our version of the workaround was missing support for textures with unnormalized coordinates, where we shouldn't divide by the texture size. The first patch fixes radeonsi, while the second fixes radv. This doesn't fix any existing tests, but I was able to hack the piglit textureGather test (it always samples at gl_FragCoord, which isn't enough to trigger the bug) to verify that it works on radeonsi, and I wrote a patch to the vulkan CTS to test this on radv.