[x86] Improve and correct how the new vector shuffle lowering was
matching and lowering 64-bit insertions.

The first problem was that we weren't looking through bitcasts to
discover that we *could* lower as insertions. Once fixed, we in turn
weren't looking through bitcasts to discover that we could fold a load
into the lowering. Once fixed, we weren't forming a SCALAR_TO_VECTOR
node around the inserted element and instead were passing a scalar to
a DAG node that expected a vector. It turns out there are some patterns
that will "lower" this into the correct asm, but the rest of the X86
backend is very unhappy with such antics.

This should fix a few more edge case regressions I've spotted going
through the regression test suite to enable the new vector shuffle
lowering.