AArch64: don't be too greedy when folding :lo12: accesses into mem ops.

This frequently leads to cases like:
   ldr xD, [xN, :lo12:var]
   add xA, xN, :lo12:var
   ldr xD, [xA, #8]

where the ADD would have been needed anyway, and the two distinct addressing
modes can prevent the formation of an ldp. Because of how we handle ADRP
(aggressively forming an ADRP/ADD pseudo-inst at ISel time), this pattern also
results in duplicated ADRP instructions (one on its own to cover the ldr, and
one combined with the add).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223172 91177308-0d34-0410-b5e6-96231b3b80d8
Use a typed enum instead of 'unsigned char' for packed field.  NFC.

This makes it easier to debug Twine as the 'Kind' fields now show their enum values in lldb and not escaped characters.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223178 91177308-0d34-0410-b5e6-96231b3b80d8
AArch64: strengthen Darwin ABI alignment assumptions

A global variable without an explicit alignment specified should be assumed to
be ABI-aligned according to its type, like on other platforms. This allows us
to use better memory operations when accessing it.

rdar://18533701

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223180 91177308-0d34-0410-b5e6-96231b3b80d8