target/alpha: Don't corrupt error_code with unknown softfloat flags

target/alpha: Don't corrupt error_code with unknown softfloat flags

In do_cvttq() we set env->error_code with what is supposed to be a
set of FPCR exception bit values.  However, if the set of float
exception flags we get back from softfloat for the conversion
includes a flag which is not one of the three we expect here
(invalid_cvti, invalid, inexact) then we will fall through the
if-ladder and set env->error_code to the unconverted softfloat
exception_flag value.  This will then cause us to take a spurious
exception.

This is harmless now, but when we add new floating point exception
flags to softfloat it will cause problems.  Add an else clause to the
if-ladder to make it ignore any float exception flags it doesn't care
about.

Specifically, without this fix, 'make check-tcg' will fail for Alpha
when the commit adding float_flag_input_denormal_used lands.

Fixes: aa3bad5b59e7 ("target/alpha: Use float64_to_int64_modulo for CVTTQ")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>