tcg-s390: Use load-address for addition

Since we're always in 64-bit mode, load address performs a full
64-bit add.  Use that for 3-address addition, as well as for
larger constant addends when we lack extended-immediates facility.

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-s390: Use all 20 bits of the offset in tcg_out_mem

This can save one insn, if the constant has any bits in 32-63 set,
but no bits in 21-31 set.  It never results in more insns.

Signed-off-by: Richard Henderson <rth@twiddle.net>