tcg: update README about size changing ops

few specific operations must be implemented to allow it (see add2_i32,

sub2_i32, brcond2_i32).

On a 64 bit target, the values are transfered between 32 and 64-bit

registers using the following ops:

- trunc_shr_i64_i32

- ext_i32_i64

- extu_i32_i64

They ensure that the values are correctly truncated or extended when

moved from a 32-bit to a 64-bit register or vice-versa. Note that the

trunc_shr_i64_i32 is an optional op. It is not necessary to implement

it if all the following conditions are met:

- 64-bit registers can hold 32-bit values

- 32-bit values in a 64-bit register do not need to stay zero or

  sign extended

- all 32-bit TCG ops ignore the high part of 64-bit registers

Floating point operations are not supported in this version. A

previous incarnation of the code generator had full support of them,

but it is better to concentrate on integer operations first.

On a 64 bit target, no assumption is made in TCG about the storage of

the 32 bit values in 64 bit registers.

4.2) Constraints

GCC like constraints are used to define the constraints of every