Fix out-of-bounds access in nested select/if_then_else codegen

[LeiWang1999]

Summary

• Fix potential out-of-bounds memory access when if_then_else is nested inside select
• Add select_condition_stack_ to track outer select conditions during code generation
• Combine outer select condition with inner if_then_else condition to guard memory access

Problem

When generating code for select(cond1, if_then_else(cond2, A[index], default), fallback), the previous implementation would evaluate A[index] even when cond1 is false, potentially causing out-of-bounds access.

Before:

float condval;
if (cond2) {
    condval = A[index];  // Executed even when cond1=false!
} else {
    condval = default;
}
result = (cond1 ? condval : fallback);

After:

float condval;
if ((cond1) && (cond2)) {
    condval = A[index];  // Only executed when cond1=true
} else if (cond1) {
    condval = default;
}
result = (cond1 ? condval : fallback);

Test plan

• Tested with select_oob.py example that triggers OOB when index < 0
• Verified generated CUDA code correctly guards memory access

🤖 Generated with Claude Code