Handle code gen for the unreachable instruction if it's the only instruction in

Handle code gen for the unreachable instruction if it's the only instruction in
the function. We'll just turn it into a "trap" instruction instead.

The problem with not handling this is that it might generate a prologue without
the equivalent epilogue to go with it:

$ cat t.ll
define void @foo() {
entry:
  unreachable
}
$ llc -o - t.ll -relocation-model=pic -disable-fp-elim -unwind-tables
        .section        __TEXT,__text,regular,pure_instructions
        .globl  _foo
        .align  4, 0x90
_foo:                                   ## @foo
Leh_func_begin0:
## BB#0:                                ## %entry
        pushq   %rbp
Ltmp0:
        movq    %rsp, %rbp
Ltmp1:
Leh_func_end0:
...

The unwind tables then have bad data in them causing all sorts of problems.

Fixes <rdar://problem/8096481>.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@108473 91177308-0d34-0410-b5e6-96231b3b80d8