[x86/SLH] Fix a bug where we would try to post-load harden non-GPRs.

Found cases that hit the assert I added. This patch factors the validity
checking into a nice helper routine and calls it when deciding to harden
post-load, and asserts it when doing so later.

I've added tests for the various ways of loading a floating point type,
as well as loading all vector permutations. Even though many of these go
to identical instructions, it seems good to somewhat comprehensively
test them.

I'm confident there will be more fixes needed here, I'll try to add
tests each time as I get this predicate adjusted.

llvm-svn: 337160