tcg: implement real ext_i32_i64 and extu_i32_i64 ops

tcg: implement real ext_i32_i64 and extu_i32_i64 ops

Implement real ext_i32_i64 and extu_i32_i64 ops. They ensure that a
32-bit value is always converted to a 64-bit value and not propagated
through the register allocator or the optimizer.

Cc: Andrzej Zaborowski <[email protected]>
Cc: Alexander Graf <[email protected]>
Cc: Blue Swirl <[email protected]>
Cc: Stefan Weil <[email protected]>
Acked-by: Claudio Fontana <[email protected]>
Signed-off-by: Aurelien Jarno <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>