accel/tcg: Add a quicker check for breakpoints

accel/tcg: Add a quicker check for breakpoints

Profiling QEMU during Fedora 35 for PPC64 boot revealed that a
considerable amount of time was being spent in
check_for_breakpoints() (0.61% of total time on PPC64 and 2.19% on
amd64), even though it was just checking that its queue was empty
and returning, when no breakpoints were set. It turns out this
function is not inlined by the compiler and it's always called by
helper_lookup_tb_ptr(), one of the most called functions.

By leaving only the check for empty queue in
check_for_breakpoints() and moving the remaining code to
check_for_breakpoints_slow(), called only when the queue is not
empty, it's possible to avoid the call overhead. An improvement of
about 3% in total time was measured on POWER9.

Signed-off-by: Leandro Lupori <leandro.lupori@eldorado.org.br>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20221025202424.195984-2-leandro.lupori@eldorado.org.br>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>