It is clearly profitable to propagate the first insn into the second
one and get:
(set (reg:FPRX2 66) (mem/c:FPRX2 (reg/v:DI 62)))
fwprop actually manages to perform this, but doesn't think the result is
worth it, which results in unnecessary store/load sequences on s390.
Improve the situation by classifying SUBREG -> MEM changes as
profitable.