target/i386: split eflags computation out of gen_compute_eflags

target/i386: split eflags computation out of gen_compute_eflags

The new x86 decoder wants the gen_* functions to compute EFLAGS before
writeback, which can be an issue for instructions with a memory
destination such as ARPL or shifts.

Extract code to compute the EFLAGS without clobbering CC_SRC, in case
the memory write causes a fault.  The flags writeback mechanism will
take care of copying the result to CC_SRC.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>