tcg/optimize: allow constant to have copies

Now that copies and constants are tracked separately, we can allow
constant to have copies, deferring the choice to use a register or a
constant to the register allocation pass. This prevent this kind of
regular constant reloading:

-OUT: [size=338]
+OUT: [size=298]
   mov    -0x4(%r14),%ebp
   test   %ebp,%ebp
   jne    0x7ffbe9cb0ed6
   mov    $0x40002219f8,%rbp
   mov    %rbp,(%r14)
-  mov    $0x40002219f8,%rbp
   mov    $0x4000221a20,%rbx
   mov    %rbp,(%rbx)
   mov    $0x4000000000,%rbp
   mov    %rbp,(%r14)
-  mov    $0x4000000000,%rbp
   mov    $0x4000221d38,%rbx
   mov    %rbp,(%rbx)
   mov    $0x40002221a8,%rbp
   mov    %rbp,(%r14)
-  mov    $0x40002221a8,%rbp
   mov    $0x4000221d40,%rbx
   mov    %rbp,(%rbx)
   mov    $0x4000019170,%rbp
   mov    %rbp,(%r14)
-  mov    $0x4000019170,%rbp
   mov    $0x4000221d48,%rbx
   mov    %rbp,(%rbx)
   mov    $0x40000049ee,%rbp
   mov    %rbp,0x80(%r14)
   mov    %r14,%rdi
   callq  0x7ffbe99924d0
   mov    $0x4000001680,%rbp
   mov    %rbp,0x30(%r14)
   mov    0x10(%r14),%rbp
   mov    $0x4000001680,%rbp
   mov    %rbp,0x30(%r14)
   mov    0x10(%r14),%rbp
   shl    $0x20,%rbp
   mov    (%r14),%rbx
   mov    %ebx,%ebx
   mov    %rbx,(%r14)
   or     %rbx,%rbp
   mov    %rbp,0x10(%r14)
   mov    %rbp,0x90(%r14)
   mov    0x60(%r14),%rbx
   mov    %rbx,0x38(%r14)
   mov    0x28(%r14),%rbx
   mov    $0x4000220e60,%r12
   mov    %rbx,(%r12)
   mov    $0x40002219c8,%rbx
   mov    %rbp,(%rbx)
   mov    0x20(%r14),%rbp
   sub    $0x8,%rbp
   mov    $0x4000004a16,%rbx
   mov    %rbx,0x0(%rbp)
   mov    %rbp,0x20(%r14)
   mov    $0x19,%ebp
   mov    %ebp,0xa8(%r14)
   mov    $0x4000015110,%rbp
   mov    %rbp,0x80(%r14)
   xor    %eax,%eax
   jmpq   0x7ffbebcae426
   lea    -0x5f6d72a(%rip),%rax        # 0x7ffbe3d437b3
   jmpq   0x7ffbebcae426

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Richard Henderson <rth@twiddle.net>
