[ARM] Don't Enable AES Pass for Generic Cores

This brings clang/llvm into line with GCC. The Pass is still enabled for
the affected cores, but is now opt-in when using `-march=`.

I also took the opportunity to add release notes for this change.

Reviewed By: john.brawn

Differential Revision: https://reviews.llvm.org/D125775

clang/docs/ReleaseNotes.rst

@@ -265,6 +265,11 @@ New Compiler Flags
   the parameter list were ``void``. There is no ``-fknr-functions`` or
   ``-fno-no-knr-functions`` flag; this feature cannot be disabled in language
   modes where it is required, such as C++ or C2x.
+- A new ARM pass to workaround Cortex-A57 Erratum 1742098 and Cortex-A72 Erratum
+  1655431 can be enabled using ``-mfix-cortex-a57-aes-1742098`` or
+  ``-mfix-cortex-a72-aes-1655431``. The pass is enabled when using either of
+  these cpus with ``-mcpu=`` and can be disabled using
+  ``-mno-fix-cortex-a57-aes-1742098`` or ``-mno-fix-cortex-a72-aes-1655431``.
 
 Deprecated Compiler Flags
 -------------------------

llvm/docs/ReleaseNotes.rst

@@ -99,6 +99,8 @@ Changes to the ARM Backend
   warnings will be generated and -mrestrict-it is now always off by default.
   Previously it was on by default for Armv8 and off for all other architecture
   versions.
+* Added a pass to workaround Cortex-A57 Erratum 1742098 and Cortex-A72
+  Erratum 1655431. This is enabled by default when targeting either CPU.
 
 Changes to the AVR Backend
 --------------------------

llvm/lib/Target/ARM/ARM.td

@@ -1161,7 +1161,7 @@ include "ARMScheduleM7.td"
 // ARM processors
 //
 // Dummy CPU, used to target architectures
-def : ProcessorModel<"generic",     CortexA8Model,      [FeatureFixCortexA57AES1742098]>;
+def : ProcessorModel<"generic",     CortexA8Model,      []>;
 
 // FIXME: Several processors below are not using their own scheduler
 // model, but one of similar/previous processor. These should be fixed.