Merge and improve code that detects same value in comparisons.

-Wtautological-overlap-compare and self-comparison from -Wtautological-compare
relay on detecting the same operand in different locations.  Previously, each
warning had it's own operand checker.  Now, both are merged together into
one function that each can call.  The function also now looks through member
access and array accesses.

Differential Revision: https://reviews.llvm.org/D66045

llvm-svn: 372453