[llvm-exegesis] Do not try to assign random registers twice.

[courbet]

Summary

Doing a random assignment assigns both tested (forward) and back-to-back
(backward) instructions.

When none of the tested instruction and back-to-back instruction have
implicit aliasing, we're currently trying to do a random register
asignment twice.

Fix this (see PR26418).

[gchatelet]

Good catch! Thx a lot.

llvm/test/tools/llvm-exegesis/X86/latency-IN16rr.s

+# RUN: llvm-exegesis -mode=latency -opcode-name=IN16rr -repetition-mode=duplicate | FileCheck %s
+
+CHECK:      ---
+CHECK-NEXT: mode: latency
+CHECK-NEXT: key:
+CHECK-NEXT:   instructions:
+CHECK-NEXT:     - 'IN16rr'
+CHECK-NEXT:     - {{.*}}
+CHECK-NEXT: config: ''
+CHECK-NEXT: register_initial_values:
+CHECK-DAG: - '[[REG1:[A-Z0-9]+]]=0x0'
+CHECK-LAST: ...

llvm/tools/llvm-exegesis/lib/SerialSnippetGenerator.cpp

case ExecutionMode::SERIAL_VIA_NON_MEMORY_INSTR: {
     for (const auto *OtherInstr : computeAliasingInstructions(
              State, &Instr, kMaxAliasingInstructions, ForbiddenRegisters)) {
       const AliasingConfigurations Forward(Instr, *OtherInstr);
       const AliasingConfigurations Back(*OtherInstr, Instr);
       InstructionTemplate ThisIT(Variant);
       InstructionTemplate OtherIT(OtherInstr);
       if (!Forward.hasImplicitAliasing())
         setRandomAliasing(Forward, ThisIT, OtherIT);
-      if (!Back.hasImplicitAliasing())
+      else if (!Back.hasImplicitAliasing())
         setRandomAliasing(Back, OtherIT, ThisIT);
       CodeTemplate CT;
       CT.Execution = ExecutionModeBit;
       CT.Info = std::string(ExecutionClassDescription);
       CT.Instructions.push_back(std::move(ThisIT));
       CT.Instructions.push_back(std::move(OtherIT));
       CodeTemplates.push_back(std::move(CT));
     }