tcg/optimize: Stop forcing z_mask to "garbage" for 32-bit values

tcg/optimize: Stop forcing z_mask to "garbage" for 32-bit values

This "garbage" setting pre-dates the addition of the type
changing opcodes INDEX_op_ext_i32_i64, INDEX_op_extu_i32_i64,
and INDEX_op_extr{l,h}_i64_i32.

So now we have a definitive points at which to adjust z_mask
to eliminate such bits from the 32-bit operands.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Luis Pires <luis.pires@eldorado.org.br>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>