[TableGen] Make more use of findSubRegIdx. NFCI.

Outside of CodeGenRegisters itself, we only want to find existing
SubRegIdxs, not create new ones. Change findSubRegIdx to assert and use
it consistently for this purpose.

Author: jayfoad

llvm/utils/TableGen/Common/CodeGenRegisters.cpp

@@ -1333,7 +1333,7 @@ CodeGenSubRegIndex *CodeGenRegBank::getSubRegIdx(const Record *Def) {
 
 const CodeGenSubRegIndex *
 CodeGenRegBank::findSubRegIdx(const Record *Def) const {
-  return Def2SubRegIdx.lookup(Def);
+  return Def2SubRegIdx.at(Def);
 }
 
 CodeGenRegister *CodeGenRegBank::getReg(const Record *Def) {

llvm/utils/TableGen/DAGISelMatcherGen.cpp

@@ -730,7 +730,6 @@ void MatcherGen::EmitResultLeafAsOperand(const TreePatternNode &N,
       // 7 bit and we cannot use StringInteger.
       if (RB.getSubRegIndices().size() > 127) {
         const CodeGenSubRegIndex *I = RB.findSubRegIdx(Def);
-        assert(I && "Cannot find subreg index by name!");
         if (I->EnumValue > 127) {
           AddMatcher(new EmitIntegerMatcher(I->EnumValue, MVT::i32,
                                             NextRecordedOperandNo));

llvm/utils/TableGen/GlobalISelEmitter.cpp

@@ -1333,7 +1333,7 @@ Error GlobalISelEmitter::importLeafNodeRenderer(
     }
 
     if (R->isSubClassOf("SubRegIndex")) {
-      const CodeGenSubRegIndex *SubRegIndex = CGRegs.getSubRegIdx(R);
+      const CodeGenSubRegIndex *SubRegIndex = CGRegs.findSubRegIdx(R);
       MIBuilder.addRenderer<ImmRenderer>(SubRegIndex->EnumValue);
       return Error::success();
     }
@@ -1606,7 +1606,8 @@ Expected<action_iterator> GlobalISelEmitter::importExplicitUseRenderers(
     if (!SubRegInit)
       return failedImport("EXTRACT_SUBREG child #1 is not a subreg index");
 
-    CodeGenSubRegIndex *SubIdx = CGRegs.getSubRegIdx(SubRegInit->getDef());
+    const CodeGenSubRegIndex *SubIdx =
+        CGRegs.findSubRegIdx(SubRegInit->getDef());
     const TreePatternNode &ValChild = Dst.getChild(0);
     if (!ValChild.isLeaf()) {
       // We really have to handle the source instruction, and then insert a
@@ -1675,7 +1676,8 @@ Expected<action_iterator> GlobalISelEmitter::importExplicitUseRenderers(
 
       if (const DefInit *SubRegInit =
               dyn_cast<DefInit>(SubRegChild.getLeafValue())) {
-        CodeGenSubRegIndex *SubIdx = CGRegs.getSubRegIdx(SubRegInit->getDef());
+        const CodeGenSubRegIndex *SubIdx =
+            CGRegs.findSubRegIdx(SubRegInit->getDef());
 
         if (Error Err = importNodeRenderer(M, DstMIBuilder, ValChild, InsertPt))
           return Err;
@@ -2004,7 +2006,7 @@ const CodeGenRegisterClass *GlobalISelEmitter::inferSuperRegisterClass(
   const DefInit *SubRegInit = dyn_cast<DefInit>(SubRegIdxNode.getLeafValue());
   if (!SubRegInit)
     return nullptr;
-  const CodeGenSubRegIndex *SubIdx = CGRegs.getSubRegIdx(SubRegInit->getDef());
+  const CodeGenSubRegIndex *SubIdx = CGRegs.findSubRegIdx(SubRegInit->getDef());
 
   // Use the information we found above to find a minimal register class which
   // supports the subregister and type we want.
@@ -2034,7 +2036,7 @@ const CodeGenSubRegIndex *GlobalISelEmitter::inferSubRegIndexForNode(
   const DefInit *SubRegInit = dyn_cast<DefInit>(SubRegIdxNode.getLeafValue());
   if (!SubRegInit)
     return nullptr;
-  return CGRegs.getSubRegIdx(SubRegInit->getDef());
+  return CGRegs.findSubRegIdx(SubRegInit->getDef());
 }
 
 Expected<RuleMatcher> GlobalISelEmitter::runOnPattern(const PatternToMatch &P) {