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>.

llvm-svn: 108473