target-i386: optimize flags checking after sub using CC_SRCT

After a comparison or subtraction, the original value of the LHS will
currently be reconstructed using an addition.  However, in most cases
it is already available: store it in a temp-local variable and save 1
or 2 TCG ops (2 if the result of the addition needs to be extended).

The temp-local can be declared dead as soon as the cc_op changes again,
or also before the translation block ends because gen_prepare_cc will
always make a copy before returning it.  All this magic, plus copy
propagation and dead-code elimination, ensures that the temp local will
(almost) never be spilled.

Example (cmp $0x21,%rax + jbe):

 Before                                     After
----------------------------------------------------------------------------
 movi_i64 tmp1,$0x21                        movi_i64 tmp1,$0x21
 movi_i64 cc_src,$0x21                      movi_i64 cc_src,$0x21
 sub_i64 cc_dst,rax,tmp1                    sub_i64 cc_dst,rax,tmp1
 add_i64 tmp7,cc_dst,cc_src
 movi_i32 cc_op,$0x11                       movi_i32 cc_op,$0x11
 brcond_i64 tmp7,cc_src,leu,$0x0            discard loc11
                                            brcond_i64 rax,cc_src,leu,$0x0

 Before                                     After
----------------------------------------------------------------------------
  mov    (%r14),%rbp                        mov    (%r14),%rbp
  mov    %rbp,%rbx                          mov    %rbp,%rbx
  sub    $0x21,%rbx                         sub    $0x21,%rbx
  lea    0x21(%rbx),%r12
  movl   $0x11,0xa0(%r14)                   movl   $0x11,0xa0(%r14)
  movq   $0x21,0x90(%r14)                   movq   $0x21,0x90(%r14)
  mov    %rbx,0x98(%r14)                    mov    %rbx,0x98(%r14)
  cmp    $0x21,%r12                     |   cmp    $0x21,%rbp
  jbe    ...                                jbe    ...

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
