re PR rtl-optimization/6305 (c++ gets ICE in reload_cse_simplify_operands)

PR optimization/6305
* config/s390/s390.c (s390_expand_plus_operand): Use find_replacement
to make sure previous reloads are taken into account.  Generate
better code if one operand is an in-range immediate constant.

From-SVN: r52417