Change the other half of aliasGEP (which handles GEP differencing) to use DecomposeGE...

Change the other half of aliasGEP (which handles GEP differencing) to use DecomposeGEPExpression.  This dramatically simplifies and shrinks the code by eliminating the horrible CheckGEPInstructions method, fixes a miscompilation (@test3) and makes the code more aggressive.  In particular, we now handle the @test4 case, which is reduced from the SmallPtrSet constructor.  Missing this caused us to emit a variable length memset instead of a fixed size one.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@89922 91177308-0d34-0410-b5e6-96231b3b80d8