[LV] Allow assume calls in predicated blocks.

The assume intrinsic is intentionally marked as may reading/writing
memory, to avoid passes moving them around. When flattening the CFG
for predicated blocks, we have to drop the assume calls, as they
are control-flow dependent.

There are some cases where we can do better (when control flow is
preserved), but that is follow-up work.

Fixes PR43620.

Reviewers: hsaito, rengolin, dcaballe, Ayal

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D68814