Pretending that the source is i64 when it is in fact i32 is
incorrect; we have type-changing opcodes that must be used.
This bug trips up the subsequent change to the optimizer.

Fixes: 4f2331e5
Reviewed-by: Luis Pires <luis.pires@eldorado.org.br>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>