Fix Thumb __builtin_longjmp with integrated assembler. <rdar://problem/11203543>

The tLDRr instruction with the last register operand set to the zero register
prints in assembly as if no register was specified, and the assembler encodes
it as a tLDRi instruction with a zero immediate.  With the integrated assembler,
that zero register gets emitted as "r0", so we get "ldr rx, [ry, r0]" which
is broken.  Emit the instruction as tLDRi with a zero immediate.  I don't
know if there's a good way to write a testcase for this.  Suggestions welcome.

Opportunities for follow-up work:
1) The asm printer should complain if a non-optional register operand is set
   to the zero register, instead of silently dropping it.
2) The integrated assembler should complain in the same situation, instead of
   silently emitting the operand as "r0".

llvm-svn: 154261