tcg: don't keep dead outputs in registers

tcg: don't keep dead outputs in registers

If an op with dead outputs is not removed, because it has side effects
or has multiple output and only one dead, mark the registers as dead
instead of saving them. This avoid a few register spills on TCG targets
with low register count, especially with div2 and mul2 ops, or when a
qemu_ld* result is not used (prefetch emulation for example).

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>