[Bug target/79354] [7 Regression] -mcpu=power8 -O2 generates power9 instruction on powerpc64le-linux

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79354

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Looking at tmp-mddump.md, the only other insn that emits the stxssp instruction
is movsf_hardfloat and that uses wb constraint for the input_operand and only
uses wu constraint for stxsspx, while movsi_from_sf uses wu constraint for both
stxssp and stxsspx.
Using wb instead of wu for movsi_from_sf constraint on the stxssp alternative
fixes the bug.