[TableGen] Fix register class handling in TableGen's DAG ISel Matcher Generator

[vhscampos]

Summary

In TableGen's instruction selection table generator, references to
register classes were handled by generating a matcher table entry in the
form of "EmitStringInteger, MVT::i32, 'RegisterClassID'". This ID is in
fact the enum integer value corresponding to the register class.

However, both the table generator and the table consumer
(SelectionDAGISel) assume that this ID is less than or equal to 127,
i.e. at most 7 bits. Values greater than this threshold cause completely
wrong behaviours in the instruction selection process.

This patch adds a check to determine if the enum integer value is
greater than the limit of 127. In finding so, the generator emits an
"EmitInteger" instead, which properly supports values with arbitrary
sizes.

Commit f8d044bbcfdc9e1ddc02247ffb86fe39e1f277f0 fixed the very same bug
for register subindices. The present patch now extends this cover to
register classes.

[rampitec]

LGTM, but needs a test in the llvm/test/TableGen/.

[vhscampos]

Added a test.

[rampitec]

LGTM

llvm/utils/TableGen/DAGISelMatcherGen.cpp

if (Def->getName() == "undef_tied_input") {
       return;
     }
 
     // Handle a reference to a register class. This is used
     // in COPY_TO_SUBREG instructions.
     if (Def->isSubClassOf("RegisterOperand"))
       Def = Def->getValueAsDef("RegClass");
     if (Def->isSubClassOf("RegisterClass")) {
+      // If the register class has an enum integer value greater than 127, the
+      // encoding overflows the limit of 7 bits, which precludes the use of
+      // StringIntegerMatcher. In this case, fallback to using IntegerMatcher.
+      const CodeGenRegisterClass &RC =
+          CGP.getTargetInfo().getRegisterClass(Def);
+      if (RC.EnumValue <= 127) {
         std::string Value = getQualifiedName(Def) + "RegClassID";
         AddMatcher(new EmitStringIntegerMatcher(Value, MVT::i32));
         ResultOps.push_back(NextRecordedOperandNo++);
+      } else {
+        AddMatcher(new EmitIntegerMatcher(RC.EnumValue, MVT::i32));
+        ResultOps.push_back(NextRecordedOperandNo++);
+      }
       return;
     }
 
     // Handle a subregister index. This is used for INSERT_SUBREG etc.
     if (Def->isSubClassOf("SubRegIndex")) {
       const CodeGenRegBank &RB = CGP.getTargetInfo().getRegBank();
       // If we have more than 127 subreg indices the encoding can overflow
       // 7 bit and we cannot use StringInteger.