Fix v1 instrumentation breaking if-else chains

[kingkingkingson]

Summary

• Fix instrument_v1 inserting distance_instrument() before else / else if, which breaks the if-else chain and fails compilation with else without a previous if.
• When the target line starts with else, insert the call inside the else block (after {) instead.

Test plan

• Instrument a TCU whose loc points at an else if line (e.g. libming decompile.c CVE-2018-8962) and confirm the generated source compiles.
• Confirm non-else target lines still get instrumentation inserted before the statement.
• Build v1 binary via trigfuzz.driver --use-script --skip-fuzz on the swftocxx target.

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