This removes all of the problems with unaligned accesses
to the bytecode stream.

With an 8-bit opcode at the bottom, we have 24 bits remaining,
which are generally split into 6 4-bit slots.  This fits well
with the maximum length opcodes, e.g. INDEX_op_add2_i32, which
have 6 register operands.

We have, in previous patches, rearranged things such that there
are no operations with a label which have more than one other
operand.  Which leaves us with a 20-bit field in which to encode
a label, giving us a maximum TB size of 512k -- easily large.

Change the INDEX_op_tci_movi_{i32,i64} opcodes to tci_mov[il].
The former puts the immediate in the upper 20 bits of the insn,
like we do for the label displacement.  The later uses a label
to reference an entry in the constant pool.  Thus, in the worst
case we still have a single memory reference for any constant,
but now the constants are out-of-line of the bytecode and can
be shared between different moves saving space.

Change INDEX_op_call to use a label to reference a pair of
pointers in the constant pool.  This removes the only slightly
dodgy link with the layout of struct TCGHelperInfo.

The re-encode cannot be done in pieces.

Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>