[llvm-mca] Improve barriers for strict region marking (PR52198)

[RKSimon]

Summary

As suggested on the bug, to help (but not completely....) stop folded instructions crossing the inline asm barriers used for llvm-mca analysis, we should recommend tagging with memory captures/attributes.

[andreadb]

LGTM. Thanks!

llvm/docs/CommandGuide/llvm-mca.rst

 cannot have the same name.
 
 There is no support for marking regions from high-level source code, like C or
 C++. As a workaround, inline assembly directives may be used:
 
 .. code-block:: c++
 
   int foo(int a, int b) {
-    __asm volatile("# LLVM-MCA-BEGIN foo");
+    __asm volatile("# LLVM-MCA-BEGIN foo":::"memory");
     a += 42;
-    __asm volatile("# LLVM-MCA-END");
+    __asm volatile("# LLVM-MCA-END":::"memory");
     a *= b;
     return a;
   }
 
 However, this interferes with optimizations like loop vectorization and may have
 an impact on the code generated. This is because the ``__asm`` statements are
 seen as real code having important side effects, which limits how the code
 around them can be transformed. If users want to make use of inline assembly