[x86] Actually initialize the SLH pass with the x86 backend and use

a shorter name ('x86-slh') for the internal flags and pass name.

Without this, you can't use the -stop-after or -stop-before
infrastructure. I seem to have just missed this when originally adding
the pass.

The shorter name solves two problems. First, the flag names were ...
really long and hard to type/manage. Second, the pass name can't be the
exact same as the flag name used to enable this, and there are already
some users of that flag name so I'm avoiding changing it unnecessarily.

llvm-svn: 339836