[Bug libfortran/80602] Reduce stack usage for blocked matmul

[tkoenig at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80602

--- Comment #4 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
Might want to backport the 8.0 patch to gcc-7, but only
after the dust from the regressions this caused has settled.