Add some shortcuts in LazyValueInfo to reduce compile time of
Correlated Value Propagation.

The patch is to partially fix PR10584. Correlated Value Propagation
queries LVI to check non-null for pointer params of each callsite. If
we know the def of param is an alloca instruction, we know it is
non-null and can return early from LVI. Similarly, CVP queries LVI to
check whether pointer for each mem access is constant. If the def of
the pointer is an alloca instruction, we know it is not a constant
pointer. These shortcuts can reduce the cost of CVP significantly.

Differential Revision: https://reviews.llvm.org/D18066