rtl: ICE with thread_local and inline asm [PR104777]

In r270550, Jakub fixed classify_insn to handle asm goto: if the asm can
jump to a label, the insn should be a JUMP_INSN.

However, as the following testcase shows, non-null ASM_OPERANDS_LABEL_VEC
doesn't guarantee that the rtx has any actual labels it can branch to.
Here, the rtvec has 0 elements because expand_asm_stmt created it:

  rtvec labelvec = rtvec_alloc (nlabels); // nlabels == 0

This causes an ICE in update_br_prob_note: BRANCH_EDGE (bb) crashes
because there's no branch edge.  I think we can fix this by checking
that there is at least one label the asm can jump to before wrapping
the ASM_OPERANDS in a JUMP_INSN.

	PR rtl-optimization/104777

gcc/ChangeLog:

	* rtl.cc (classify_insn): For ASM_OPERANDS, return JUMP_INSN only if
	ASM_OPERANDS_LABEL_VEC has at least one element.

gcc/testsuite/ChangeLog:

	* gcc.dg/torture/tls/pr104777.c: New test.