i965/fs: Use MOV_INDIRECT for all indirect uniform loads

i965/fs: Use MOV_INDIRECT for all indirect uniform loads

Instead of using reladdr, this commit changes the FS backend to emit a
MOV_INDIRECT whenever we need an indirect uniform load.  We also have to
rework some of the other bits of the backend to handle this new form of
uniform load.  The obvious change is that demote_pull_constants now acts
more like a lowering pass when it hits a MOV_INDIRECT.