[GlobalISel] Fix and tidy up documentation for ValueMapping class (NFC)

[ehjogab]

Summary

The documentation was missing a '*/' in '/*<2x32-bit> vadd {0, 64, VPR}',
and the example code are now aligned to improve readability.

[bjope]

llvm/include/llvm/CodeGen/GlobalISel/RegisterBankInfo.h

107

A bit unfortunate to get too long lines here. Not sure if that is worse compared to things not being aligned in the code comments.

[bjope]

Should I land this patch for you (including a fix for the long lines)?

llvm/include/llvm/CodeGen/GlobalISel/RegisterBankInfo.h

 //===- llvm/CodeGen/GlobalISel/RegisterBankInfo.h ---------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
public:
   /// E.g.,
   /// Let say we have a 32-bit add and a <2 x 32-bit> vadd. We
   /// can expand the
   /// <2 x 32-bit> add into 2 x 32-bit add.
   ///
   /// Currently the TableGen-like file would look like:
   /// \code
   /// PartialMapping[] = {
-  /// /*32-bit add*/    {0, 32, GPR}, // Scalar entry repeated for first vec elt.
-  /// /*2x32-bit add*/  {0, 32, GPR}, {32, 32, GPR},
-  /// /*<2x32-bit> vadd {0, 64, VPR}
+  /// /*32-bit add*/      {0, 32, GPR}, // Scalar entry repeated for first vec elt.
+  /// /*2x32-bit add*/    {0, 32, GPR}, {32, 32, GPR},
+  /// /*<2x32-bit> vadd*/ {0, 64, VPR}
   /// }; // PartialMapping duplicated.
   ///
   /// ValueMapping[] {
-  ///   /*plain 32-bit add*/ {&PartialMapping[0], 1},
+  ///   /*plain 32-bit add*/       {&PartialMapping[0], 1},
   ///   /*expanded vadd on 2xadd*/ {&PartialMapping[1], 2},
-  ///   /*plain <2x32-bit> vadd*/ {&PartialMapping[3], 1}
+  ///   /*plain <2x32-bit> vadd*/  {&PartialMapping[3], 1}
   /// };
   /// \endcode
   ///
   /// With the array of pointer, we would have:
   /// \code
   /// PartialMapping[] = {
-  /// /*32-bit add lower */ {0, 32, GPR},
+  /// /*32-bit add lower */ { 0, 32, GPR},
   /// /*32-bit add upper */ {32, 32, GPR},
-  /// /*<2x32-bit> vadd {0, 64, VPR}
+  /// /*<2x32-bit> vadd */  { 0, 64, VPR}
   /// }; // No more duplication.
   ///
   /// BreakDowns[] = {
-  /// /*AddBreakDown*/ &PartialMapping[0],
+  /// /*AddBreakDown*/   &PartialMapping[0],
   /// /*2xAddBreakDown*/ &PartialMapping[0], &PartialMapping[1],
-  /// /*VAddBreakDown*/ &PartialMapping[2]
+  /// /*VAddBreakDown*/  &PartialMapping[2]
   /// }; // Addresses of PartialMapping duplicated (smaller).
   ///
   /// ValueMapping[] {
-  ///   /*plain 32-bit add*/ {&BreakDowns[0], 1},
+  ///   /*plain 32-bit add*/       {&BreakDowns[0], 1},
   ///   /*expanded vadd on 2xadd*/ {&BreakDowns[1], 2},
-  ///   /*plain <2x32-bit> vadd*/ {&BreakDowns[3], 1}
+  ///   /*plain <2x32-bit> vadd*/  {&BreakDowns[3], 1}
   /// };
   /// \endcode
   ///
   /// Given that a PartialMapping is actually small, the code size
   /// impact is actually a degradation. Moreover the compile time will
   /// be hit by the additional indirection.
   /// If PartialMapping gets bigger we may reconsider.
   struct ValueMapping {