mesa/prog: relative offsets into constbufs are not constant

The optimization logic relies on being able to read out constbuf values
from program parameters. However that only works if there's no relative
addressing involved.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91173
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.5 10.6" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>

src/mesa/program/prog_opt_constant_fold.c

@@ -38,6 +38,8 @@ src_regs_are_constant(const struct prog_instruction *inst, unsigned num_srcs)
    for (i = 0; i < num_srcs; i++) {
       if (inst->SrcReg[i].File != PROGRAM_CONSTANT)
 	 return false;
+      if (inst->SrcReg[i].RelAddr)
+         return false;
    }
 
    return true;