Fix opt crash in addVectorPasses when TargetMachine is null

[gmh5225]

Summary

• Null-check TM in PassBuilder::addVectorPasses before calling getTargetTriple(), matching the existing Windows guards at the call sites.
• Prevents opt -passes=default<O1> from access-violating (0xC0000005) when no target triple / TargetMachine is available (e.g. empty IR input).

Closes #174

Test plan

• echo '' | opt -passes='default<O1>' -disable-output should exit cleanly instead of crashing
• With a Windows triple / TargetMachine present, vector passes remain disabled as before
• With a non-Windows TargetMachine, vector passes still run

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