AArch64: remove arm64 triple enumerator.

Having both Triple::arm64 and Triple::aarch64 is extremely confusing, and
invites bugs where only one is checked. In reality, the only legitimate
difference between the two (arm64 usually means iOS) is also present in the OS
part of the triple and that's what should be checked.

We still parse the "arm64" triple, just canonicalise it to Triple::aarch64, so
there aren't any LLVM-side test changes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213743 91177308-0d34-0410-b5e6-96231b3b80d8
[ARM] Make the assembler reject unpredictable pre/post-indexed ARM STR instructions.

The ARM ARM prohibits STR instructions with writeback into the source register. With this commit this constraint is now enforced and we stop assembling STR instructions with unpredictable behavior.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213745 91177308-0d34-0410-b5e6-96231b3b80d8