fix: do not update last_opcode_pos when emitting OP_debug

[G-Yong]

OP_debug is a transparent no-op (0 pop, 0 push) used solely for debugger breakpoints. The previous code in emit_source_loc() set last_opcode_pos to point at OP_debug:

fd->last_opcode_pos = bc->size;  // ← pointed at OP_debug
dbuf_putc(bc, OP_debug);

This breaks all peephole optimizations that rely on get_prev_opcode(), because they see OP_debug instead of the real preceding opcode.

Affected code paths

Function	Impact
js_is_live_code()	Misidentifies dead code as live (OP_debug doesn't match any terminal opcode)
set_object_name()	Fails to match OP_set_name / OP_set_class_name, skips name assignment
set_object_name_computed()	Same as above for computed names
lvalue parsing (js_parse_unary)	Falls into default → "invalid lvalue" error

This caused CI test failures when OP_debug was always emitted (after removing the emit_debug flag).

Fix

Remove fd->last_opcode_pos = bc->size; from emit_source_loc(), making OP_debug transparent to the peephole optimizer — just like OP_source_loc already is. Neither opcode updates last_opcode_pos.