target/i386: avoid using s->tmp0 for add to implicit registers

target/i386: avoid using s->tmp0 for add to implicit registers

For updates to implicit registers (RCX in LOOP instructions, RSI or RDI
in string instructions, or the stack pointer) do the add directly using
the registers (with no temporary) if 32-bit or 64-bit, or use a temporary
created for the occasion if 16-bit.  This is more efficient and removes
move instructions for the MO_TL case.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Link: https://lore.kernel.org/r/20241215090613.89588-14-pbonzini@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>