[LV] Don't sink scalar instructions that may read from memory.

The current sinking code doesn't prevent us from sinking a load past an
aliasing store. Skip sinking instructions that may read from memory to
avoid a mis-compile.

See @minimal_bit_widths_with_aliasing_store for an example where 2 loads
are sunk past aliasing stores before this fix.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D147259