The testcase from PR47086

void
foo ()
{
  int n = 0;
  while (1)
    {
      int i[n % 1];
      n++;
    }
}

reveals that while on the tree level n++ has an outgoing EH edge, RTL
expansion leaves us with

;; n_5 = n_1 + 1;

(insn 12 11 13 (set (reg:DI 67)
        (sign_extend:DI (reg/v:SI 61 [ n ]))) /space/rguenther/src/svn/trunk/gcc/testsuite/gcc.dg/pr47086.c:11 -1
     (nil))

(insn 13 12 14 (set (reg:DI 4 si)
        (const_int 1 [0x1])) /space/rguenther/src/svn/trunk/gcc/testsuite/gcc.dg/pr47086.c:11 -1
     (nil))

(insn 14 13 15 (set (reg:DI 5 di)
        (reg:DI 67)) /space/rguenther/src/svn/trunk/gcc/testsuite/gcc.dg/pr47086.c:11 -1
     (nil))

(call_insn/u 15 14 16 (set (reg:DI 0 ax)
        (call (mem:QI (symbol_ref:DI ("__addvdi3") [flags 0x41]) [0 S1 A8])
            (const_int 0 [0]))) /space/rguenther/src/svn/trunk/gcc/testsuite/gcc.dg/pr47086.c:11 -1
     (expr_list:REG_EH_REGION (const_int -2147483648 [0xffffffff80000000])
        (nil))
    (expr_list:REG_DEP_TRUE (use (reg:DI 4 si))
        (expr_list:REG_DEP_TRUE (use (reg:DI 5 di))
            (nil))))

(insn 16 15 17 (set (reg:DI 68)
        (reg:DI 0 ax)) /space/rguenther/src/svn/trunk/gcc/testsuite/gcc.dg/pr47086.c:11 -1
     (expr_list:REG_EQUAL (plus:DI (reg:DI 67)
            (const_int 1 [0x1]))
        (nil)))

(insn 17 16 0 (set (reg/v:SI 63 [ n ])
        (subreg:SI (reg:DI 68) 0)) /space/rguenther/src/svn/trunk/gcc/testsuite/gcc.dg/pr47086.c:11 -1
     (nil))

note that the call is marked CONST.  The EH edge is simply thrown away:

;; Basic block 4 , prev 3, next 5, loop_depth 1, count 0, freq 10000, maybe hot, flags: new reachable rtl modified.
;; Predecessors:  3 [100.0%]  (fallthru) 5 [100.0%]  (dfs_back)
...
(call_insn/u 15 14 16 4 (set (reg:DI 0 ax)
        (call (mem:QI (symbol_ref:DI ("__addvdi3") [flags 0x41]) [0 S1 A8])
            (const_int 0 [0]))) /space/rguenther/src/svn/trunk/gcc/testsuite/gcc.dg/pr47086.c:11 -1
     (expr_list:REG_EH_REGION (const_int -2147483648 [0xffffffff80000000])
        (nil))
    (expr_list:REG_DEP_TRUE (use (reg:DI 4 si))
        (expr_list:REG_DEP_TRUE (use (reg:DI 5 di))
            (nil))))

(insn 16 15 17 4 (set (reg:DI 68)
        (reg:DI 0 ax)) /space/rguenther/src/svn/trunk/gcc/testsuite/gcc.dg/pr47086.c:11 -1
     (expr_list:REG_EQUAL (plus:DI (reg:DI 67)
            (const_int 1 [0x1]))
        (nil)))

(insn 17 16 18 4 (set (reg/v:SI 63 [ n ])
        (subreg:SI (reg:DI 68) 0)) /space/rguenther/src/svn/trunk/gcc/testsuite/gcc.dg/pr47086.c:11 -1
     (nil))

;; Successors:  5 [100.0%]  (fallthru)


So we seem to leave around the libcall because of the EH side-effect
but do not consider that call as possibly throwing (as it is marked
nothrow on RTL) in find_many_sub_basic_blocks
(at least) and DCE it later, too (sounds similar to PR19020).

Currently generated assembly for gcc.dg/pr47086.c on x86_64 is

foo:
.LFB0:
        .cfi_startproc
        .cfi_personality 0x3,__gcc_personality_v0
        pushq   %rbx
        .cfi_def_cfa_offset 16
        .cfi_offset 3, -16
        movl    $0, %ebx
.L2:
        addl    $1, %ebx
        jmp     .L2


I'm running into this issue when removing unreachable blocks during
expansion, then the landing pad unwind info expansion ICEs because the landing
pad was removed.

We mark the call as not throwing via emit_libcall_block because we pass
it (plus (reg) 1) as EQUIV, which may not trap.