[GlobalISel] Update the documentation and comment for G_[UN]MERGE_VALUES

In r296921, we added the G_[UN]MERGE_VALUES node, but did not update the
documentation. Fixing that.

NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314168 91177308-0d34-0410-b5e6-96231b3b80d8

docs/GlobalISel.rst

@@ -304,6 +304,11 @@ As opposed to SelectionDAG, there are no legalization phases.  In particular,
 Legalization is iterative, and all state is contained in GMIR.  To maintain the
 validity of the intermediate code, instructions are introduced:
 
+* ``G_MERGE_VALUES`` --- concatenate multiple registers of the same
+  size into a single wider register.
+
+* ``G_UNMERGE_VALUES`` --- extract multiple registers of the same size
+  from a single wider register.
 
 * ``G_EXTRACT`` --- extract a simple register (as contiguous sequences of bits)
   from a single wider register.

include/llvm/Target/GenericOpcodes.td

@@ -511,6 +511,7 @@ def G_INSERT : Instruction {
   let hasSideEffects = 0;
 }
 
+/// Concatenante multiple registers of the same size into a wider register.
 def G_MERGE_VALUES : Instruction {
   let OutOperandList = (outs type0:$dst);
   let InOperandList = (ins variable_ops);