Fix compiler-explorer#5784 (compiler-explorer#5793)

Fix the name of the entry block in an IR control flow graph.

lib/cfg/cfg-parsers/llvm-ir.ts

@@ -27,6 +27,7 @@ import {BaseInstructionSetInfo} from '../instruction-sets/base.js';
 import {assert, unwrap} from '../../assert.js';
 
 export type BBRange = {
+    namePrefix: string; // used to encode the function name in the first block
     nameId: string;
     start: number;
     end: number;
@@ -79,29 +80,32 @@ export class LlvmIrCfgParser extends BaseCFGParser {
         const result: BBRange[] = [];
         let i = fn.start + 1;
         let bbStart = i;
-        let currentName = fnName;
+        let currentName: string = '';
+        let namePrefix: string = fnName + '\n\n';
         while (i < fn.end) {
             const match = code[i].text.match(this.labelRe);
             if (match) {
+                const label = match[1];
                 if (bbStart === i) {
-                    // for -emit-llvm the first basic block doesn't have a label, for the ir viewer it does though
                     assert(result.length === 0);
-                    bbStart = i + 1;
+                    currentName = label;
                 } else {
-                    const label = match[1];
                     // start is the fn / label define, end is exclusive
                     result.push({
+                        namePrefix: namePrefix,
                         nameId: currentName,
                         start: bbStart,
                         end: i,
                     });
                     currentName = label;
-                    bbStart = i + 1;
+                    namePrefix = '';
                 }
+                bbStart = i + 1;
             }
             i++;
         }
         result.push({
+            namePrefix: '',
             nameId: currentName,
             start: bbStart,
             end: i,
@@ -118,7 +122,7 @@ export class LlvmIrCfgParser extends BaseCFGParser {
             }
             return {
                 id: e.nameId,
-                label: `${e.nameId}${e.nameId.includes(':') ? '' : ':'}\n${this.concatInstructions(
+                label: `${e.namePrefix}${e.nameId}${e.nameId.includes(':') ? '' : ':'}\n${this.concatInstructions(
                     asms,
                     e.start,
                     end,