[FlattenCFG] Fix an Imprecise Usage of AA (#128117)
In current `FlattenCFG`, using `isNoAlias` for two instructions is
imprecise. For example, when passing a store instruction and a load
instruction directly into `AA->isNoAlias`, it will always return
`NoAlias`. This happens because when checking the types of the two
Values, the store instruction (which has a `void` type) causes the
analysis to return `NoAlias`.
For instructions, we should use `getModRefInfo` instead of `isNoAlias`,
as aliasing is a concept of memory locations.
In this patch, `AAResults::getModRefInfo` is supported to take in two
instructions. It will check whether two instructions may access the same
memory location or not. And in `FlattenCFG`, we use this new helper
function to do the check instead of `isNoAlias`.
Unit tests and lit tests are also included to this patch.
