Revert two GEP-related InstCombine commits

This reverts commit r216523 and r216598; people have reported
regressions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216698 91177308-0d34-0410-b5e6-96231b3b80d8
[FastISel][AArch64] Don't fold instructions that are not in the same basic block.

This fix checks first if the instruction to be folded (e.g. sign-/zero-extend,
or shift) is in the same machine basic block as the instruction we are folding
into.

Not doing so can result in incorrect code, because the value might not be
live-out of the basic block, where the value is defined.

This fixes rdar://problem/18169495.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216700 91177308-0d34-0410-b5e6-96231b3b80d8
Introduce -DLLVM_USE_SANITIZER=Undefined CMake option to build UBSan-ified version of LLVM/Clang.

I've fixed most of the simple bugs and currently "check-llvm" test suite
has 26 failures, and "check-clang" suite has 5 failures.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216701 91177308-0d34-0410-b5e6-96231b3b80d8