tcg/s390x: Tighten constraints for 64-bit compare

Give 64-bit comparison second operand a signed 33-bit immediate.
This is the smallest superset of uint32_t and int32_t, as used
by CLGFI and CGFI respectively.  The rest of the 33-bit space
can be loaded into TCG_TMP0.  Drop use of the constant pool.

Reviewed-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>