[NewGVN] Fix lifetime coercion (#141477)
Before commit 14dee0a, NewGVN would not miscompile the function foo,
because `isMustAlias` would return false for non-pointers, particularly
for `lifetime.start`. We need to check whether the loaded pointer is
defined by`lifetime.start` in order to safely simplify the load to
uninitialized memory.
For `getInitialValueOfAllocation`, the behavior depends on the
allocation function. Therefore, we take a conservative approach: we
check whether it's a pointer type. If it is, then—based on the earlier
check—we know that the allocation function defines the loaded pointer.
