[PATCH] PR59448 - Promote consume to acquire

[Andrew MacLeod]

Lengthy discussion : https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59448

Basically we can generate incorrect code for an atomic consume operation 
in some circumstances.  The general feeling seems to be that we should 
simply promote all consume operations to an acquire operation until 
there is a better definition/understanding of the consume model and how 
GCC can track it.

I proposed a simple patch in the PR, and I have not seen or heard of any 
dissenting opinion.   We should get this in before the end of stage 3 I 
think.

The problem with the patch in the PR is the  memory model is immediately 
promoted from consume to acquire.   This happens *before* any of the 
memmodel checks are made.  If a consume is illegally specified (such as 
in a compare_exchange), it gets promoted to acquire and the compiler 
doesn't report the error because it never sees the consume.

This new patch simply makes the adjustment after any errors are checked 
on the originally specified model.   It bootstraps on 
x86_64-unknown-linux-gnu and passes all regression testing.
I also built an aarch64 compiler and it appears to issue the LDAR as 
specified in the PR, but anyone with a vested interest really ought to 
check it out with a real build to be sure.

OK for trunk?

Andrew
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