[InstCombine][x86] fix insertion point bug in vector demanded elts fold (PR48476)

This transform was added at:
c63799fc

From what I see, it's the first demanded elements transform that adds
a new instruction using the IRBuilder. There are similar folds in
the generic demanded bits chunk of instcombine that also use the
InsertPointGuard code pattern.

The tests here would assert/crash because the new instruction was
being added at the start of the demanded elements analysis rather
than at the instruction that is being replaced.