[LICM] Bail if checking a global/constant for invariant.start

When we check if a load is loop invariant by finding a dominating
invariant.start call, we strip bitcasts until we get to an i8* Value,
and look for an invariant.start use of the i8* Value.

We may accidentally end up at an i8 global and look at a global's uses,
which we shouldn't do in a loop pass. Although we could make this
logic work with globals, that's not currently intended.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D111098