gallivm: use fallback code for mul_hi with llvm >= 7.0

gallivm: use fallback code for mul_hi with llvm >= 7.0

LLVM 7.0 ditched the pmulu intrinsics.
This is only a trivial patch to use the fallback code instead.
It'll likely produce atrocious code since the pattern doesn't match what
llvm itself uses in its autoupgrade paths, hence the pattern won't be
recognized.

Should fix https://bugs.freedesktop.org/show_bug.cgi?id=111496

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>