In PowerPC, the AtomicCmpXchgInst is lowered to
ISD::ATOMIC_CMP_SWAP_WITH_SUCCESS. However, this node does not handle
the weak attribute of AtomicCmpXchgInst. As a result, when compiling C++
atomic_compare_exchange_weak_explicit, the generated assembly includes a
"reservation lost" loop — i.e., it branches back and retries if the
stwcx. (store-conditional) fails. This differs from GCC’s codegen, which
does not include that loop for weak compare-exchange.

Since PowerPC uses LL/SC-style atomic instructions, the patch enables
AtomicExpandImpl::expandAtomicCmpXchg for PowerPC. With this, the weak
attribute is properly respected, and the "reservation lost" loop is
removed for weak operations.

---------

Co-authored-by: Matt Arsenault <arsenm2@gmail.com>

This makes them more consistent with the checks performed by regular loads. We can't simply add IsNonExtLoad to the existing atomic_load_8/16/32/64 as that would affect out of tree targets.

ISD::ADDC, ISD::ADDE, ISD::SUBC and ISD::SUBE are being deprecated,
using ISD::UADDO_CARRY,ISD::USUBO_CARRY instead. Lowering the UADDO,
UADDO_CARRY, USUBO, USUBO_CARRY in the patch.

This reverts commit 7763119c6eb0976e4836f81c9876c49a36d46d73 (leaving the modifications from 03cb46d248b08)..

ISD::ADDC, ISD::ADDE, ISD::SUBC and ISD::SUBE are being deprecated,
using ISD::UADDO_CARRY,ISD::USUBO_CARRY instead. Lowering the UADDO,
UADDO_CARRY, USUBO, USUBO_CARRY in the patch.

Commit 93589057830b2c3c35500ee8cac25c717a1e98f9 was reverted because it
caused a failure with test `lld :: ELF/ppc64-local-exec-tls.s`. This
relands the commit with a fix for the test.

This commit broke a test on a couple bots
lld :: ELF/ppc64-local-exec-tls.s

This reverts commit 93589057830b2c3c35500ee8cac25c717a1e98f9.

If an instruction doesn't support memory operands, but one is provided,
an error should be raised. And conversely, if an instruction requires a
memory operand, but none is given, an error should be raised.

Fixes:   https://github.com/llvm/llvm-project/issues/71030

Bug only happens in 64bit involving spills. Since we don't know when the
spill will happen, all instructions in the chain used to deduce sign
extension for eliminating 'extsw' will need to be promoted to 64-bit
pseudo instructions.

The following instruction will promoted in PPCMIPeepholes: EXTSH, LHA,
ISEL to EXTSH8, LHA8, ISEL8

These builtins are currently returning CR0 which will have the format
[0, 0, flag_true_if_saved, XER].
We only want to return flag_true_if_saved. This patch adds a shift to
remove the XER bit before returning.