Make FindAvailableLoadedValue TBAA aware

FindAvailableLoadedValue() relies on FindAvailablePtrLoadStore() to run
the alias analysis when searching for an equivalent value. However,
FindAvailablePtrLoadStore() calls the alias analysis framework with a
memory location for the load constructed from an address and a size,
which thus lacks TBAA metadata info. This commit modifies
FindAvailablePtrLoadStore() to accept an optional memory location as
parameter to allow FindAvailableLoadedValue() to create it based on the
load instruction, which would then have TBAA metadata info attached.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D99206