[InstSimplify] Treat invariant group insts as bitcasts for load operands

We can look through invariant group intrinsics for the purposes of
simplifying the result of a load.

Since intrinsics can't be constants, but we also don't want to
completely rewrite load constant folding, we convert the load operand to
a constant. For GEPs and bitcasts we just treat them as constants. For
invariant group intrinsics, we treat them as a bitcast.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D101103