ir_to_mesa: Initialize the (we never use it) abs field of Mesa src regs.

src/mesa/shader/ir_to_mesa.cpp

@@ -1401,6 +1401,7 @@ mesa_src_reg_from_ir_src_reg(ir_to_mesa_src_reg reg)
    mesa_reg.Swizzle = reg.swizzle;
    mesa_reg.RelAddr = reg.reladdr;
    mesa_reg.Negate = reg.negate;
+   mesa_reg.Abs = 0;
 
    return mesa_reg;
 }