Fix crash when printing instructions that have a metadata attached but no parent.

Fix PR24852 (crash with -debug -instcombine)

Patch by Than McIntosh <thanm@google.com>

Summary:
Add guards to the asm writer to prevent crashing
when dumping an instruction that has no basic
block.

Differential Revision: http://reviews.llvm.org/D15798

From: Than McIntosh <thanm@google.com>
llvm-svn: 257094