[ARM] Fix for 64-bit CAS expansion on ARM32 with -O0

Summary:
This patch fixes comparison of 64-bit atomic with its expected value in CMP_SWAP_64 expansion.

Currently, the low words are compared with CMP, while the high words are compared with SBC. SBC expects the carry flag to be set if CMP detects a difference. CMP might leave the carry unset for unequal arguments though if the first one is >= than the second. This might cause the comparison logic to detect false equality.

Example of the broken C++ code:
```
std::atomic<long long> at(2);

long long ll = 1;
std::atomic_compare_exchange_strong(&at, &ll, 3);
```
Even though the atomic `at` and the expected value `ll` are not equal and `atomic_compare_exchange_strong` returns `false`, `at` is changed to 3.

The patch replaces SBC with CMPEQ.

Reviewers: t.p.northover

Subscribers: aemerson, rengolin, llvm-commits, asl

Differential Revision: https://reviews.llvm.org/D27315

llvm-svn: 288433