Fix for sanitizer crash introduced in r209049

This patch fixes 3 issues introduced by r209049 that only showed up in on
the sanitizer buildbots. One was a typo in a compare. The other is a check to
confirm that the single differing value in the two incoming GEPs is the same
type. The final issue was the the IRBuilder under some circumstances would
build PHIs in the middle of the block.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209065 91177308-0d34-0410-b5e6-96231b3b80d8
MachO: Add comments describing section flags/attrs

When pruning superfluous MachO structure definitions, I chose to keep
the most generically useful which is why Support's definitions won over
the ones in MC.

However, the MC copy had some useful comments describing some of the
field values.

Bring these back to the copy in Support.  While doing this, fill in some
of the underdocumented definitions as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209066 91177308-0d34-0410-b5e6-96231b3b80d8
Revert r209049 and r209065, "Add support for combining GEPs across PHI nodes"

It broke clang selfhosting even after r209065.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209067 91177308-0d34-0410-b5e6-96231b3b80d8
Reduce abuse of default values in the GlobalAlias constructor.

This is in preparation for adding an optional offset.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209073 91177308-0d34-0410-b5e6-96231b3b80d8