prevent GCC from re-arranging two emit_insn()

[William Tambe]

How to enforce that GCC will not re-arrange two emit_insn() such that,
in the example below, insn2 will always follow insn1; ie:

emit_insn (gen_insn1 ()); // emit insn1.
emit_insn (gen_insn2 ()); // emit insn2