tcg/optimize: Use fold_masks_zs in fold_qemu_ld

tcg/optimize: Use fold_masks_zs in fold_qemu_ld

Avoid the use of the OptContext slots.

Be careful not to call fold_masks_zs when the memory operation
is wide enough to require multiple outputs, so split into two
functions: fold_qemu_ld_1reg and fold_qemu_ld_2reg.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>