Thumb add to PC is incorrectly mangled

[derekbruening]

Split from #1912

Running app_process32:

<cannot encode add>
<Source operand #2 has wrong type/size>
<Application /system/bin/app_process32 (16024) DynamoRIO usage error : instr_encode error: no encoding found (see log)>
<Usage error: instr_encode error: no encoding found (see log) (/work/dr/git/src/core/arch/arm/encode.c, line 2887)

interp: start_pc = 0x735c5592
check_thread_vm_area: pc = 0x735c5592
prepend_entry_to_fraglist: putting fragment @0x735c5592 (shared) on vmarea 0x7323f000-0x749512bc
  0x735c5592  f853 c024  ldr    (%r3,%r4,lsl 2)[4byte] -> %r12
  0x735c5596  44e7       add    %pc %r12 -> %pc
mbr exit target = 0x4e956220
end_pc = 0x735c5598

exit_branch_type=0x12 bb->exit_target=0x4e956220
bb ilist before mangling:
TAG  0x735c5592
 +0    L3              f853 c024  ldr    (%r3,%r4,lsl 2)[4byte] -> %r12
 +4    L3              44e7       add    %pc %r12 -> %pc
 +6    L4 @0x4e98520c  f7d7 bae4  b      $0x4e956220 <shared_bb_ibl_indjmp>
END 0x735c5592

bb ilist after mangling:
TAG  0x735c5592
 +0    L3              f853 c024  ldr    (%r3,%r4,lsl 2)[4byte] -> %r12
 +4    m4 @0x4e985968  f84a 0c00  str    %r0 -> (%r10)[4byte]
 +8    m4 @0x4ea76930  f245 509a  movw   $0x0000559a -> %r0
 +12   m4 @0x4ea6fdc4  f2c7 305c  movt   $0x0000735c -> %r0
 +16   m4 @0x4e985f40  f8ca 2008  str    %r2 -> +0x08(%r10)[4byte]
 +20   m4 @0x4ea7671c             <label>
ERROR: Could not find encoding for: add    %r0 %r12 -> %r2
Reason: Source operand #2 has wrong type/size
SYSLOG_ERROR: Application /system/bin/app_process32 (16177).  Internal Error: DynamoRIO debug check failure: /work/dr/git/src/core/arc
h/disassemble_shared.c:1441 nxt_pc != NULL

It's missing the shift immeds for the encoding that takes 3 registers.