tcg/optimize: Propagate sign info for setcond

The result is either 0 or 1, which means that we have
a 2 bit signed result, and thus 62 bits of sign.
For clarity, use the smask_from_zmask function.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Luis Pires <luis.pires@eldorado.org.br>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

tcg/optimize.c

@@ -1730,6 +1730,7 @@ static bool fold_setcond(OptContext *ctx, TCGOp *op)
     }
 
     ctx->z_mask = 1;
+    ctx->s_mask = smask_from_zmask(1);
     return false;
 }
 
@@ -1802,6 +1803,7 @@ static bool fold_setcond2(OptContext *ctx, TCGOp *op)
     }
 
     ctx->z_mask = 1;
+    ctx->s_mask = smask_from_zmask(1);
     return false;
 
  do_setcond_const: