aco: Use zero for MUBUF/MTBUF when soffset is undefined.

No Fossil DB changes on Rembrandt (GFX10.3).

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21930>

src/amd/compiler/aco_instruction_selection.cpp

@@ -4428,6 +4428,9 @@ mubuf_load_callback(Builder& bld, const LoadEmitInfo& info, Temp offset, unsigne
       soffset = Operand(info.soffset);
    }
 
+   if (soffset.isUndefined())
+      soffset = Operand::zero();
+
    bool offen = !vaddr.isUndefined();
    bool idxen = info.idx.id();
 
@@ -5486,6 +5489,9 @@ mtbuf_load_callback(Builder& bld, const LoadEmitInfo& info, Temp offset, unsigne
       soffset = Operand(info.soffset);
    }
 
+   if (soffset.isUndefined())
+      soffset = Operand::zero();
+
    const bool offen = !vaddr.isUndefined();
    const bool idxen = info.idx.id();