[Headers][X86] Replace \operation with \code{.operation}

\operation ... \endoperation are not valid doxygen commands and cause issues when -Wdocumentation is enabled (Issue #35297)

This patch proposes to replace them with \code{.operation} ... \endcode blocks so that the pseudo-code is correctly retained in any documentation and downstream can use the ".operation" type for its own formatting.

Differential Revision: https://reviews.llvm.org/D125170